Even assuming that AI code can't be copyrighted by the person running the AI (seems like a stretch), the company just needs to prove that someone, at some point, made a direct modification to the code not through the AI. It only takes one drop of copyright to make it a copyrighted work.

>the company just needs to prove that someone, at some point, made a direct modification to the code not through the AI. It only takes one drop of copyright to make it a copyrighted work.

Company A: You stole our code >:(

Company B: Can you tell us which part we stole?

Company A: It's almost all vibecoded, but there's one function where a developer fixed it by hand

Company B: Okay we'll rewrite that function then :^)

Yes, but to sue for infringement, you must register the work with the Copyright Office, and the registration must specify clearly what's AI and what's human-created, and only the latter is protected.