A policy like this has two points. One, to give good faith potential contributors a guideline on what the project expects. Two, to help reviewers have a clear policy they can point to to reject AI slop PRs, without feeling bad or getting into conflicts about minutiae of the code.

Right, "good faith" is a key idea that is being ignored. If you want to lie to the lead SDL maintainers and claim your code is 100% human-written, you can probably get away with it. But that is unethical and cynical behavior in pursuit of an astonishingly petty goal. And it's correct for SDL to simply ignore the contribution because it came from a dishonest developer, even if the specific code appears to be very good.