How does it matter? It either is or is not plagiarism. Ripping off a published textbook isn't somehow better than ripping off private correspondence. Both are serious acts of academic misconduct on account of the part where you knowingly and intentionally portrayed someone else's work as your own.

Note that I am not taking a stance on what openai allegedly did or did not do one way or the other. I am merely pointing out what I see as a fatal flaw in the line of argument presented by the earlier commenter - the idea that training on an item is on its own sufficient to establish plagiarism of it.