Doesn't matter, that human is still the responsible party putting it there.

The whole premise here is weird anyway; unless you're using a RAG designed to quote code, the chances that you end up in a perfectly identical environment needing exactly the same operation, AND it's proprietary somehow, AND you're the one working on it and wouldn't know there was a proprietary feature (whatever that even means here) is astronomically low.

If you were writing a book with a handful of other people and you used an LLM which PERFECTLY reproduced a chapter of another book on the topic without attribution, whose fault would that be? It's not like there are software 2 liners that are just off limits for use, so besides the fact that it would be crazy unlikely to reproduce a perfect copy of a huge block of code, the chances of you working in the Linux kernel for example, getting your commit added to the official version, and not knowing it's proprietary by looking at it is, IMO, so unlikely it's not an argument against culpability.

Maybe if you're vibe coding an app from scratch on your personal GitHub or something (even then the odds of perfect recapture are stupid low), but it's not really possible or a good argument here.

Edit: I guess the obvious exception here is if some LLM provider DID train a model to use whole pieces of code and strip attributions and licensing. In that case, I don't see how they survive getting sued into the ground.