Are they gatekeeping to protect against AI "slop" or are they gatekeeping to protect an inefficient and non-scalable review process against velocity?

I've seen policies like this in various places and they do not generally seem to be based quantitative measures of code functionality, security, and efficiency.

There are other approaches to take to deal with large numbers of incoming PRs: improved CI, AI-readable standards for AI code, better static testing, AI first-pass review, etc.

It's fine to enshrine hobbyism into your code review policy to keep things fun and human-scale. On the other hand, where projects actually matter, it is necessary to think about code review as part of an industrial process with inputs and outputs, one where this sort of thing has no place.