Specifically about "humans": my understanding is that Clean-room design is not a requirement and the "Case law" section on your Wikipedia link explains that and has examples.
How and whether the same principles can be applied to LLMs, I have no idea. I imagine it would involve discussions about creativity, for example.
Not a lawyer.
The thing that happened with "clean-room design" is that one company did it and the court said it was allowed. Companies are risk averse. Once they know one thing is yes and something else is maybe, they go with the yes, right? With the result that the supply of court cases where something other than that happened tends to dry up, leaving the other alternatives to stay as maybe.
If some people have now decided they're not going to blink in the face of a risk, we might get to find out whether some of the maybes turn out to be a yes or a no.