Sure. I don't think it's particularly likely to have been a conspiracy. Just speculating. If I were to try to put an illicit backdoor in a compiler, I would try and make damned sure it's not traceable to me.

It's a very subtle thing. You have to submit a patch with an improvement (or, at least, an apparent one) to the compiler, which also contains your backdoor. You have to ensure said patch doesn't immediately cause test failures in gcc, or in projects using gcc. If you can manage that, though, it'd be very difficult to prove that you were responsible, which is why I think the targeted attack is entirely plausible.