It doesn't really matter what your stance on AI is, the problem is the increased review burden on OSS maintainers.
In the past, the code itself was a sort of proof of effort - you would need to invest some time and effort on your PRs, otherwise they would be easily dismissed at a glance. That is no longer the case, as LLMs can quickly generate PRs that might look superficially correct. Effort can still have been out into those PRs, but there is no way to tell without spending time reviewing in more detail.
Policies like this help decrease that review burden, by outright rejecting what can be identified as LLM-generated code at a glance. That is probably a fair bit today, but it might get harder over time, though, so I suspect eventually we will see a shift towards more trust-based models, where you cannot submit PRs if you haven't been approved in advance somehow.
Even if we assume LLMs would consistently generate good enough quality code, code submitted by someone untrusted would still need detailed review for many reasons - so even in that case it would like be faster for the maintainers to just use the tools themselves, rather than reviewing someone else's use of the same tools.