Ironically though this makes the reverse a bit more defensible (i.e. using an LLM to reverse engineer a proprietary app) because that proprietary app's source code is less likely to be among the publicly available dataset.

But I imagine the corpos aren't going to look fondly on that for obvious reasons.

This really isn't true though, even if it is currently true in many cases. Case in point, if I wrote something and published it right now, it wouldn't be part of any AI model yet. A party with a lot of money (like, say, a tech corporation) could easily create a bespoke coding model that is trained on everything but the desired libraries, thus achieving "clean room".