The "clean room" aspect for that came in the way that the people writing the new implementation had no knowledge of the original source material, they were just given a specification to implement (see also Oracle v. Google).
If you're feeding an LLM GPL'd code and it "creates" something "new" from it, that's not "clean room", right?
At the end of the day the supposed reimplementation that the LLM generates isn't copyrightable either so maybe this is all moot.