And humans can introduce new vulnerabilities, too. Linux maintainers accept patches from random people over the internet. They already have to just the patch on its technical merit, not on who submitted it. For them I don't think it makes a difference (technically) if the patch was written with the help of an AI.

I believe that there may be valid reasons to refuse anything that was touched by an AI, but that's not about security. It's more about the non-technical problems coming from AI (centralising power, increasing inequalities, carbon footprint, ...).