It's not unenforceable at all. Major players would be forced to abide by it, smaller players would reduce their use of LLMs, and not marking LLM content would be a bannable offense on most platforms.
black market guns are also a thing, and they're realtivly easy to manufacture using untracked machines and materials using skills you can develop in under a year. That doesn't mean that regulating the sale / supply / ownership of guns isn't useful.