In Tafsir Al-Jalalayn, it says that Allah promised them this number at first but then sent more becoming 3000 angels then 5000 angels:
[Quran 8:9]
When you sought help from your Lord, asking Him to help you by granting you victory over them,
and He answered you [saying]: ‘I shall reinforce you, I shall assist you, with a thousand angels, rank
upon rank’, one rank following after the next: God promised them this [number] at first, but it then
became three thousand, and then five thousand, as stated in [sūrat] Āl ʿImrān [Q. 3:124-125] (a variant
reading [of alf, ‘thousand’] has the plural āluf, ‘thousands’,1 similar [in pattern] to aflus, ‘coins’).
https://ia800900.us.archive.org/26/items/AlJalalainEng/Al_Jalalain_Eng.pdf, page 187
In Tafsir Kashani, it says that it could mean what Ibn Kathir's tafsir said or that it was just expressing a great multitude of angels [involved] and not the specific number:
[Quran 8:9]
When you sought help from your Lord, declaring that your might and strength was [not from you but] from Him and turning away from the veils of your acts by attaining certainty in that [the power to] effect and strength come from Him, neither from you, nor from your enemies; and He answered, your call at the instant of that disengagement from the garb of acts and the attributes of the soul, with [the statement]: 'I shall reinforce you, from the world of angelic beings by virtue of the correspondence in terms of genus of your hearts with that [world] at that instant, with a thousand angels, with a world from the dominion of vanquishing, that is, with the heavenly faculties and their spiritual beings, which in that state [that you were now in] corresponded to your hearts, as was alluded to already in [sūrat] Āl ʿImrān [Q. 3:124-5]. As for the difference in the numbers [of angels] in these two instances, this is either because what is meant is the great multitude [involved] and not the specific number, or because His saying here: rank upon rank', indicates that they are followed by another contingent of the same [angels]. As for their being reinforced [with these angels], this is either by their becoming embodied (tajassud) and imaginalised (tamaththul) in the forms of fighters, as for example when forms are imaginalised in dreams, such that they are awed by them, or by their effective and vanquishing powers reaching them such that they are destroyed and defeated.
https://quranx.com/Tafsirs/8.9
Regardless of which tafsir (explanation) you favor, there is no contradiction.