The biggest mistake, AI or not, is dropping a 10K+ PR. 300~500 LOC is how far one should be going, unless they're doing some automated refactoring. E.g. formatting the entire StaticCompiler.jl source. This should've been a distinct PR, preferably by a maintainer.
I agree with you.
I've seen this in other places as well.
The bottleneck is not coding or creating a PR, the bottleneck is the review.