Well, unenforceable isn't a synonym for undetectable or awkward. Their policy indicates that they are aware of this difficulty: if you admit to using AI then they close your pull request, if you do not admit to using AI but evidence later surfaces that you did then they ban you. They can enforce this.
The hope here is the same hope as most laws: that lies eventually catch up to people. That truth comes to light. But sure, in the meanwhile, there are always dishonest people around trying to flout rules to varying degrees of success. Some are caught right away, some live their entire lives without it catching up to them. That doesn't make the rule unenforceable, that just highlights the limits of rules: it requires evidence that can be hard to come by.
The hope here is the same hope as most laws: that lies eventually catch up to people. That truth comes to light. But sure, in the meanwhile, there are always dishonest people around trying to flout rules to varying degrees of success. Some are caught right away, some live their entire lives without it catching up to them. That doesn't make the rule unenforceable, that just highlights the limits of rules: it requires evidence that can be hard to come by.