Why Hafidh Zubair 'Ali Zi'a declared this "hassan" is a big question
First of all the hadith you've quoted is one of the da'if narrations which declare remaining unmarried as something bad.
The reason for it weakness is the narrator 'Isa ibn Maymun (from Medina the companion of al-Qasim via whom this hadith was transmitted) عِيسَى بْنُ مَيْمُونٍ who was declared as weak by scholars such as ibn Ma'yn, ibn Hebban, abu Hathim ar-Razi and as rejected and an author of manakeer by an-Nasa-i and al-Bukhari and an author of fabricated hadith acocrding to abu Nu'ym.
Therefore this hadith can hardly be reliable. Why Zubair 'Ali Zi'a declared it hassan is rather unclear maybe he would declare it hassan lighayrihi due to the hadith I'll share as other (including earlier) scholars considered it having a Sahih witness, but they agree on its weakness. But I wonder if this technically is a good and accurate approach. As both al-Busiri شهاب الدين البوصيري died 839 a.H. (in his Misbaah az-zujaja fi Zawa'idi ibn Majah كتاب مصباح الزجاجة في زوائد ابن ماجه see here) and ibn Hajar al-'Asqalani in his Talkhees al-Hibeer (hadith #1530) تلخيص الحبير declared it as da'if. As-Sindi السندي in his commentary (Hashiat as-Sindi حاشية السندي) of Sunan ibn Majah also stated (you will find this sentence in more or less the same wording in all three linked sources):
Here and in the following I'm translating from Arabic language. As these translations are of my own take them carefully.
إسناده ضعيف لاتفاقهم على ضعف عيسى بن ميمون المديني (Source)
It's sanad is da'if as they (scholars/experts) agreed that 'isa ibn Maymun al-Madani is da'if.
All three scholars mentioned above agreed that the hadith itself has a witness in the sahih which can be regarded as the context.
As-Sindi in his commentary explains the hadith as follows:
( النكاح ) أي طلب النساء بالوجه المشروع في الدين (من سنتي ) من طريقتي التي سلكتها وسبيلي التي ندبتها (فمن لم يعمل بسنتي ) رغبة وإعراضا عنها وقلة مبالاة بها فلا يشمل الحديث من يترك النكاح لعدم تيسر المؤن أو للاشتغال بالعبادة ونحو ذلك
(Marriage) i.e. seeking a relationship women in the way that is lawful in religion (from my sunnah) from my path that I have followed and my path that I have recommended (for someone who does not act upon my Sunnah) out of desire (intentionally) and rejected it (turns away from it) and or take it with indifference. The hadith does not include one who abandons marriage because supplies are not available or to be busy with worship and so on.
The possible context of this narration and the related commentary of ibn Hajar
A possible context of this hadith snippet is mentioned in other longer versions of it. A couple of young sahabah() wanted to challenge each other and show how pious they are by refraining from eating (and fasting the whole life), by refraining from women (and remain unmarried) and by praying the night (and not sleep) etc.
Narrated Anas bin Malik:
A group of three men came to the houses of the wives of the Prophet (ﷺ) asking how the Prophet (ﷺ) worshipped (Allah), and when they were informed about that, they considered their worship insufficient and said, "Where are we from the Prophet (ﷺ) as his past and future sins have been forgiven." Then one of them said, "I will offer the prayer throughout the night forever." The other said, "I will fast throughout the year and will not break my fast." The third said, "I will keep away from the women and will not marry forever." Allah's Messenger (ﷺ) came to them and said, "Are you the same people who said so-and-so? By Allah, I am more submissive to Allah and more afraid of Him than you; yet I fast and break my fast, I do sleep and I also marry women. So he who does not follow my tradition in religion, is not from me (not one of my followers). (Sahih al-Bukhari)
When the prophet () heard their talks or it reached him he spoke to the audience without naming them and disapproved these deeds saying that they contradict his sunnah. Which means his path or way of life or practice. The term sunnah here is not referring to that which is related to fard or wajib, but to the literal meaning which is a personal choice of life and can differ from one person to another.
Ibn Hajar al-'Asqalani commented the last sentence of this hadith as follows in his fath al-Bary فتح الباري -see here-:
قوله ( فمن رغب عن سنتي فليس مني ) المراد بالسنة الطريقة لا التي تقابل الفرض ، والرغبة عن الشيء الإعراض عنه إلى غيره ،
The statement (So he who does not follow my tradition in religion, is not from me) What is meant by the "sunnah" here is the path (way of life), not the one that corresponds to the obligation "fard", and not following something is to turn away from it to something else.
والمراد من ترك طريقتي وأخذ بطريقة غيري فليس مني ، ولمح بذلك إلى طريق الرهبانية فإنهم الذين ابتدعوا التشديد كما وصفهم الله تعالى وقد عابهم بأنهم ما وفوه بما التزموه ،
What is meant is that he who left my way and adopted someone else’s way is not from me, and he alluded to this in the way of monasticism, for they are the ones who invented strictness and going to extremes in religious matters as Allah Almighty described them, and HE blamed them for not fulfilling what they committed.
وطريقة النبي صلى الله عليه وسلم الحنيفية السمحة فيفطر ليتقوى على الصوم وينام ليتقوى على القيام ويتزوج لكسر الشهوة وإعفاف النفس وتكثير النسل .
And the way of the Prophet Allah's prayers and blessings be upon him is the tolerant Hanafyyah, so he breaks his fast to be strong in (next/later) fasting, sleeps to be strong in standing (in the night prayer), and gets married to break lust, chaste the nafs, and increase offspring.
وقوله فليس مني إن كانت الرغبة بضرب من التأويل يعذر صاحبه فيه فمعنى " فليس مني " أي على طريقتي ولا يلزم أن يخرج عن الملة وإن كان إعراضا وتنطعا يفضي إلى اعتقاد أرجحية عمله فمعنى فليس مني ليس على ملتي لأن اعتقاد ذلك نوع من الكفر .
And his saying, “is not from me.” If not following it is of a kind of interpretation which excuses the person, then the meaning of “it is not from me,” i.e. according to my way, and does not have to deviate from the religion. But if it is reluctance and deviation that leads to a belief that his action is more righteous, he is not from me means is not according to my religion, because belief in that is a kind of unbelief.
وفي الحديث دلالة على فضل النكاح والترغيب فيه ، وفيه تتبع أحوال الأكابر للتأسي بأفعالهم وأنه إذا تعذرت معرفته من الرجال جاز استكشافه من النساء ، وأن من عزم على عمل بر واحتاج إلى إظهاره حيث يأمن الرياء لم يكن ذلك ممنوعا .
And in the hadith is an indication of the virtue of marriage and the encouragement of it. And in it follows the wisdom of the elders to follow their actions and that if it is not possible to know it from men, it is permissible to explore it from women. Further it indicates that whoever resolves to do a good deed and needs to show it where it is safe from showing off, that is not forbidden.
We further know that it is generally the sunnah (way of life) of the prophets () to get married and therefore this is highly recommended.
Allah said in this context:
Then We sent following their footsteps Our messengers and followed [them] with Jesus, the son of Mary, and gave him the Gospel. And We placed in the hearts of those who followed him compassion and mercy and monasticism, which they innovated; ... (57:27)
But nevertheless a person (male or female) could choose to stay lonely and among the most prominent scholars imam an-Nawawi is known to have remained unmarried.
Further note that none of the reports pretending that remaining unmarried is sinful or bad was declared authentic by the hadith scholars! See also islamqa #96977 (available in Arabic, Urdu and Indonesian).
What is the point in the hadith?
The point here is that these young sahabah() wanted to go to extremes in religious matters and this is what the prophet () was disapproving as he said in another hadith:
... ‘O people, beware of exaggeration in religious matters for those who came before you were doomed because of exaggeration in religious matters.’”
(Sunan ibn Majah and Sunan an-Nasa-i)
And Allah ordered his prophet () and therefore us saying:
So remain on a right course as you have been commanded, [you] and those who have turned back with you [to Allah ], and do not transgress. Indeed, He is Seeing of what you do. (11:112)
And Allah the Almighty also said informing us that going to extremes and declaring lawful things unlawful is wrong:
Say, "Who has forbidden the adornment of Allah which He has produced for His servants and the good [lawful] things of provision?" Say, "They are for those who believe during the worldly life [but] exclusively for them on the Day of Resurrection." Thus do We detail the verses for a people who know. (7:32)
Therefore as long as you haven't declared your opposition to the sunnah of the prophet(), even if you choose to do otherwise in specific matters you have not committed any sin. On the other hand if you were told such and such is sunnah and you said something "I dislike it" this would be an issue.
What is meant by sunnah in the hadith is "the whole package" which refers to our prophet's() religiousness and he () was not among those going to extremes as the hadith (of al-Bukhari) shows when saying:
... I am more submissive to Allah and more afraid of Him than you; yet I fast and break my fast, I do sleep and I also marry women. So he who does not follow my tradition in religion, is not from me (not one of my followers)
You may find a statement of 'Abdullah ibn 'Amr ibn al-'Aas () who was one of these sahabah in Is there a hadith describing the importance of sleep?.
See also the following fatwa islamqa #103889 or islamqa.org(Hanafi) fatwa #157435.
See also: Abdullah's answer on Is it permitted to not get married?