Without context or a proper citation it is difficult to guess what is being referred to. From a cursory search the closest statement of Qurtubi with this wording which I could find in his Tafsir is regarding verse 4:94.
This verse was revealed about the following incident:
عن ابن عباس، قال مر رجل من بني سليم على نفر من أصحاب رسول الله صلى الله عليه وسلم ومعه غنم له فسلم عليهم قالوا ما سلم عليكم إلا ليتعوذ منكم فقاموا فقتلوه وأخذوا غنمه فأتوا بها رسول الله صلى الله عليه وسلم . فأنزل الله تعالى ( يا أيها الذين آمنوا إذا ضربتم في سبيل الله فتبينوا ولا تقولوا لمن ألقى إليكم السلام لست مؤمنا )
"A man from Banu Sulaim who had some sheep with him, passed by some of the Companions of the Messenger of Allah (ﷺ). He gave Salam to them and they said: 'He did not give Salam except to protect himself.' So they attacked him, killed him, and took his sheep. They went to the Messenger of Allah (ﷺ) with them, and Allah, Most High, revealed: O you who believe! When you go in the cause of Allah, verify and say not to anyone who greets you: "You are not a believer (4:94).
— Jami at-Tirmidhi also see Bukhari 4591 and Muslim 3025
Qurtubi in his tafsir on the verse (under point five) writes:
والمسلم إذا لقي الكافر ولا عهد له جاز له قتله، فإن قال: لا إله إلا الله لم يجز قتله، لأنه قد اعتصم بعصام الإسلام المانع من دمه وماله وأهله: فإن قتله بعد ذلك قتل به
When a Muslim meets a Kafir who has no covenant, then it is permissible for him to kill him. If he says "لا إله إلا الله" then it is not permitted to kill him because his Islam has made sacred his blood, property and family. If he kills him after this then the murderer will be killed in retaliation.
— Tafsir al-Qurtubi
A non-muslim with no covenant is neither a dhimmi, nor a musta’min nor a mu‘aahid - but rather is a Harbi. A Harbi can be killed since there is war with them and war permits killing. This is rather agreed upon and not something unique to Qurtubi:
اتفق الفقهاء على أن دم الكافر الحربي - وهو غير الذمي والمعاهد والمؤمن - مهدر، فإن قتله مسلم فلا تبعة عليه إذا كان مقاتلا، أما إذا كان الكافر الحربي غير مقاتل كالنساء والصبيان والعجزة والرهبان وغيرهم ممن ليسوا أهلا للقتال أو لتدبيرها فلا يجوز قتله
The jurists are agreed that the blood of a Kafir Harbi -and he is one who is other than a dhimmi, mu‘aahid or believer- is permissible, hence if he is one who can fight and a Muslim kills him then nothing is due on the killer, however if the Harbi is not one who can fight, such as a child, woman, old person or monk etc. who is not eligible to fight then their killing is not permissible
— Mawsoo‘ah al-Fiqhiyyah