IME it is much, much more difficult to review code that is the result of a LLM regurgitation than to review human-written code made by someone even a bit experienced in programming - even more so if the human in question is a domain expert.
For one, LLMs tend to waffle and produce code that works but is much more extensive and/or unintuitively organized than code that a human writes. The bigger thing however is testing. LLM code tends to test itself, whereas a domain expert human tends to have many more domain-specific edge cases in mind.
Maybe, yeah. But isn't possible for a human to write "bad code", too?
Does it matter if it comes from a human you don't know or an AI you don't know? If it's difficult to review for the maintainers, then the maintainers should not merge it.
Let me put it this way: if a random contributor opens a PR as a human, then the maintainers will do what they were doing before AI, right? Now if a random contributor opens a PR pretending to be written manually, but actually written with the help of an AI, what will the maintainers do? Exactly the same, right? Try to read and understand the contribution, and merge it if they want to take responsibility of it.
The difference, really, is that everyone and their dog can now open a PR that looks evolved, and for maintainers it's difficult because it adds noise.