Difficulty of enforcing is a detail. Since the rule exists, it can be used when detection is done. And importantly it means that ignoring the rule means you’re intentionally defrauding the project.
IIRC Mitchell Hashimoto recently proposed some system of attestations for OSS contributors. It’s non-obvious how you’d scale this.
They can spin up LLM-backed contributors faster than you can ban them.
Hence why banning AI contributions is meaningless, you literally only punish 'good' actors.