> There’s legal precedent is that you can use a derived work to generate a new unique work (the spec derived for the copyrighted code is very much a derived work)

Indeed, but in the clean room scenario, the party who implements the spec has to be a separate entity that has never seen the code. Whether or not the LLM is copyright infringing is a separate question - it definitely has (at least some) familiarity with the code in question, which makes the "clean room" argument an uphill battle