There is a lot of legal history of doing new designs with influence from reverse engineered sources. The basic rule is that you can't have someone familiar with the original code involved with writing the new code.

LLMs are a new area, but I would expect that feeding the original source into an LLM context and saying "Write this, but different" wouldn't qualify as a clean-room design. There isn't enough general removal from the original subject matter.

  Not content to destroy one potential competitor, USL also seeks to keep a stranglehold on the entire market by claiming that everyone who has had ever had access to UNIX source code a group that include~ most experienced programmers in the industry is "mentally contaminated" and may not develop any functionally similar product, without first purchasing an exorbitantly expensive license from USL.  In accordance with this argument, USL requests this Court to  prohibit BSDI from hiring anyone who has had access to UNIX source code. USL's request would not only require BSDI to fire all of its  current employees, but would make it difficult to find any qualified replacements. Having chosen to widely disseminate its source code in order to build a market for its product and having succeeded in making it an industry standard, USL cannot now claim that everyone with knowledge of it is restricted from developing competing products.

The difference is that if humans do it, there's plausible deniability that it is derived.

I mean, even when humans do it, we still do clean room engineering.

It goes further though: LLMs are just a tool, and tools don't get rights, humans do.

Humans having the right to learn from a few dozen books and projects is not the same as a tool that infers output using all copyrighted works know to man.