Why would the clause be unenforceable? It doesn’t violate any of the general principles of contract law. If you intentionally contract around these terms that don’t violate any existing body of law and don’t run counter to public interest, a court would have no problem enforcing the terms of a contract. They probably wouldn’t sue you or me in our individual capacity if we circumvented. There’s a much greater chance of recovery if they go after a company which is pretty clearly using their service in a bad faith. If ByteDance wanted to use their LLM to train their own, they could’ve negotiated such a license.