It's partly about Netflix getting sued by someone claiming infringement, but also partly (maybe mostly) about Netflix maintaining their right to sue others for infringement.
The scenario looks like this:
* Be Netflix. Own some movie or series where the main elements (plot, characters, setting) were GenAI-created.
* See someone else using your plot/characters/setting in their own for-profit works.
* Try suing that someone else for copyright infringement.
* Get laughed out of court because the US Copyright Office has already said that GenAI is not copyrightable. [1]
In a legal case? You question the authors under oath, subpoena communications records, billing records, etc.
If there's even a hint that you used AI output in the work and you failed to disclose it to the US Copyright Office, they can cancel your registration.
Other than that, just a bit of common sense tells you all you need to know about where the data comes from (datasets never released, outputs of the LLMs suspisciously close to original copyrighted content, AI founders openly saying that paying for copyrighted content is too costly etc. etc. etc.)
How does anyone prove it though? You can say "does that matter?" but once everybody starts doing it, it becomes a different story.