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.