People can upload code that violates copyright too. No llm required.
So if that is the concern have a rule about not violating copyright (which they may already have). And there is likely already a process for handling that.
This is the problem I have with the situation: the solution chosen (banning llm code) doesn't really address the concerns they've raised directly. It's just apologetics to make it sound like it's reason-based rather than "we don't like it".
It's their platform, they can do what they want, but let's not pretend it's not code-puritanism.