if you've worked with a proprietary codebase at all, you can not be let even close to a reimplementation or it's legally not clean-room
That is incorrect though. The idea behind clean room is in the name: if you take even the same idea, write a spec, and SOMEONE ELSE does a different implementation, you're fine. The someone else here is AI. And no, AI output is not patentable nor IP infringement, unless there is a case I missed something. Adobe is extremely litigious, so they haven't made a move yet. They have no case.