the "cannot be enforced" argument always felt like concern tolling to me. for things like licences, anyone can just copy code from other incompatible projects and claim it's theirs. every policy can be abused and i don't see why this is different.
if someone is hell bent on adding ai code (or any other code, like incompatible licenced code or even backdoors) they will have to make signifficant effort to mask the signals to bypass the review. for everything else it will be a good guideline on how to act when that situation happens instead of depending on the individual maintainers opinion