If the usage was very clearly fair use, there'd be no need to be defensive about it; the case could be dismissed trivially. In reality, the question would need to be sorted out in court.
Questions of derivative works and fair use come up fairly frequently even in the open source world. This isn't solely a question of corporate lawyer posturing. I don't know any copyleft authors that would be okay with someone copying & pasting their code, making trivial changes, and saying it isn't a derivative work. Of course, their understanding of the law may be flawed. You'll get to find out in court.
You're right. A lot of this boils down to how much you want to spend in court proving your usage is just under fair use. We've moved beyond the question of ethics if you're intentionally violating a project's source license and relying on fair use to do whatever you want with the code. If you want to poke someone with a stick, you can't be surprised when they hit back. I contend what the OP described isn't clearly fair use (note I'm not saying that it clearly isn't fair use either). It ultimately doesn't impact me because I'm just not going to copy & paste code from projects without attribution and following the license, but I'd be worried about anyone reading that comment as objectively true.
