> A major project will discover that it has merged a lot of AI-generated code
After a code review, at least the reviewer should know the feature well enough to maintain it. This is, at least in my experience, the main part of the job of the reviewer at the time of review: Understand what the code does, why it does it, how it does it, such that you agree with it as if it's code you've written.
If major projects merge code because "lgtm" is taken literally, then they have been merging bogus code before LLMs.