I work for a large enterprise firm, our corporate lawyer has told be about this exact scenario so I'm inclined to believe it's real.

That being said, for established projects it won't be that hard to prove the non-AI bit because you have a long commit history that predates the tooling.

Even if you were to assume that all commits after a certain date were AI generated, the OP is slightly off in their attestation that any AI code suddenly makes the whole thing public domain, it would only be if a majority of the codebase was AI coded (and provably so).

So yes all the vibe coded shite is a lost cause, but stuff like Windows isn't in any danger.

There was a case in which a monkey took a picture and the owner of the camera wanted to publish the photo. Peta sued and lost because an animal can't hold any copyright as an human author is required for copyright.

https://en.wikipedia.org/wiki/Monkey_selfie_copyright_dispute

As you also find in the wikipedia article, this case is used to argue that ai generated content is not by an human author and consequently not copyrightable.

If there was an actual civil suit you'd probably be able to subpoena people for that information, and the standard is only more likely than not. I have no idea if the general idea is bullshit, though.

IANAL