I did not expect people to take "API" so literally. This point is what I was referring to when I said "I am simplifying slightly". The point is that a clean room impl begins from a specification of what the software does, and that the new implementation is purported to be derived only from this. What I am trying to say is that "not looking at the implementation" is not exactly the point of the test - that is a rule of thumb, which works quite well for avoiding copyright infringement, but only when humans do it.
It probably works great for a machine too at least when it comes to a closed source product. The issue is specifically the part where the LLM was almost certainly trained on the code in question which is going to be an issue for any code published to the internet.