Provenance has never been something you can guarantee with code in an open contribution model (which is not the only way of governing an open source project). For all you know the code could be copy and pasted from a proprietary codebase, or be the product of third party who's work is being plagiarised by the submitter. A software project is ultimately a community and is build on trust and faith that people are acting in good faith. The kind of person who would deliberately (and gleefully as I've seen in some social media posts) go into a community and violate its stated values and standards, no matter how arbitrary and illegitimate they feel those are, has bigger character flaws to worry about than their reliance on stochastic parrots to write code.

'No AI' rules are ultimately about fostering certain community values and norms than guaranteeing no LLM code makes it in.

"it's impossible to ensure that no one pisses in the pool, therefore we're allowing pissing into the pool now"

"we can't 100% enforce that studenst don't cheat on exams, so it's not worth to take a stand against cheating"

The sloppers are literally outing themselves, it's not rocket science.