There seems to be a mistaken impression in that the WGA is advocating for a blanket ban on AI/LLMs. It isn't.
From: WGA Would Allow Artificial Intelligence in Scriptwriting - Variety
So "undetectable" usage of AI-generated material by a writer isn't an issue for the WGA. What they are arguing against is the AI can't be treated as a writer by studios - that is, studios can't present a writer with material from AI and tell them to edit it, and then screw them out of a writing credit to save money.