First of all the matter in fact looks differently, when one reads both ahadith in Arabic.
As in imam Muslim's version it is stated:
In the following I'll be translating from Arabic language, as these translations are of my own take them carefully!
... عَنْ عُبَيْدِ اللَّهِ بْنِ مِقْسَمٍ، أَنَّهُ نَظَرَ إِلَى عَبْدِ اللَّهِ بْنِ عُمَرَ كَيْفَ يَحْكِي رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ
... from 'Ubayd Allah ibn Miqsam, who was regarding (looking at) ibn 'Umar's depiction on how the Messenger of Allah, Allah's prayers and blessing be upon him was saying (narrating):
So in fact 'Abdullah ibn 'Umar not only reported what the prophet() did say, but also what he () did while saying it, by gesture.
This in my humble opinion was incompletely translated in the hadith translation provided on sunnah.com.
As for the content both ahadith are rather similar and imam Muslim quoted many versions of the same hadith and the sanad of both ahadith deviate from Abu Hazim who narrated from 'Ubayd Allah ibn Miqsam upwards.
Further be aware that a translation is always a matter of interpretation therefore for example the translation from Sunan ibn Majah -apparently- was based on a scholarly interpretation which includes information that scholars have deduced from the text, which actually is not necessarily explicitly quoted in the original. So it is often a question of on what basis a translator has translated: literal translation, translation based on approved interpretation or translation based on one's own understanding ...
Moreover what the prophet() made and said and ibn 'Umar reported and showed actually described in the qur'an as an attribute of Allah the Almighty:
They have not appraised Allah with true appraisal, while the earth entirely will be [within] His grip on the Day of Resurrection, and the heavens will be folded in His right hand. Exalted is He and high above what they associate with Him. (39:67)
So from a Salafi -literal meaning- perspective the gesture of the prophet () and ibn 'Umar is only a human description of what Allah is doing. Therefore both interpretations seem valid: The prophet () did so, and Allah the Almighty does.
And this verse is the main focus of the part of the chapter in Sahih Muslim in which this hadith was compiled.
Interpretation of the hadith
As for the interpretation:
Most scholars said that:
and clenched his hand and started to open ... it
is a description of what the prophet () has done on the Minbar.
As for those who said it is Allah it is an interpretation of His power and capability.
Here a quote from imam an-Nawawi's commentary on Sahih Muslim quoting from other scholars. Note that an-Nawawi here made a commentary including all the ahadith in this first part of the chapter starting by #2785 and ending with our hadith #2788c:
قال العلماء : المراد بقوله : ( يقبض أصابعه ويبسطها ) النبي صلى الله عليه وسلم ، ولهذا قال : إن ابن مقسم نظر إلى ابن عمر كيف يحكي رسول الله صلى الله عليه وسلم ؟
The scholars said: Who is meant by saying: (He clenches his fingers and opens them) is the Prophet, may Allah bless him and grant him peace, and for this it is said: Ibn Miqsam looked at ibn Umar how the Messenger of Allah, may Allah bless him and grant him peace, was speaking.
وأما إطلاق اليدين لله تعالى فمتأول على القدرة ، وكنى عن ذلك باليدين ، لأن أفعالنا تقع باليدين ، فخوطبنا بما نفهمه ، ليكون أوضح وأوكد في النفوس ، وذكر اليمين والشمال حتى يتم المثال ، لأنا نتناول باليمين ما نكرمه ، وبالشمال ما دونه ولأن اليمين في حقنا يقوى لما لا يقوى له الشمال ، ومعلوم أن السموات أعظم من الأرض ، فأضافها إلى اليمين ، والأرضين إلى الشمال ، ليظهر التقريب في الاستعارة ، وإن كان الله سبحانه وتعالى لا يوصف بأن شيئا أخف عليه من شيء ، ولا أثقل من شيء ، هذا مختصر كلام المازري في هذا .
As for releasing the hands to Allah Almighty, it is interpreted as power, and the usage of the two hands, is because our actions fall into the two hands, so we (humans) were addressed with what we understand, to make it clearer and assured for the souls. And the the right and the left were mentioned to finalize the example (given), because we deal with the right what we honor, and with the left what is below it, and because the right in our case strengthens what the left fails to be strong enough for. It is (further) known that the heavens are greater than the earth, so he added them to the right, and the two earths to the left, so that the example becomes clear in the metaphor. However for Allah the Almighty the description that anything is lighter for Him than something, nor that it is heavier than something fits. This is a summary of the words of the al-Maaziri on this.
قال القاضي : وفي هذا الحديث ثلاثة ألفاظ : يقبض ، ويطوي ، ويأخذ كله بمعنى الجمع لأن السماوات مبسوطة ، والأرضين مدحوة ، وممدودة ، ثم يرجع ذلك إلى معنى الرفع والإزالة وتبديل الأرض غير الأرض والسموات فعاد كله إلى ضم بعضها إلى بعض ورفعها وتبديلها بغيرها ، قال : وقبض النبي صلى الله عليه وسلم أصابعه وبسطها تمثيل لقبض هذه المخلوقات وجمعها بعد بسطها ، وحكاية للمبسوط والمقبوض ، وهو السماوات والأرضون ، لا إشارة إلى القبض والبسط الذي هو صفة القابض والباسط سبحانه وتعالى ، ولا تمثيل لصفة الله تعالى السمعية المسماة باليد التي ليست بجارحة .
Al-Qadi said: There are three terms in this hadith: to clench, fold, and take. The whole of them appear in the meaning of gathering because the heavens are unfold, and the two earths are spread (See 79:30), and elongated (See 15:19), then it goes back to the meaning of lifting and removing, and changing the earth other than the earth and the heavens, so it all returned to joining some of them together, lifting them and replacing them with others. He (further) said: The Prophet, may Allah’s prayers and peace be upon him, grasped his fingers and spread them out as a representation of the grasping of these creatures and collecting them after they were laid, and an attribute of the one who made things flat (Al-Qaabid) and the one who made things unfold (Al-Baasit), which is the heavens and the earth, there is no reference to the grasp and the extension which is the attribute of the gripper and the basin, may He be glorified and exalted, nor is it a representation of the auditory attribute of Allah Almighty called the hand that is not an organ (human like).
Finally the quotation of scholars here seemingly reflects the view of the Asha'arites as al-Maaziri, An-Nawawi and al-Qadi 'Iyad (assuming the quote is referring to him) are counted among them.