I agree that we're both right. I agree with Bingowings that time travel makes the distinction a little more fluid. If you're arguing in a strict, timeline sense, then, yes, Salvation is a sequel because it takes place farther on the timeline. But I think if you're talking in a narrative sense, which I think is more important, Salvation is very much a prequel because it tells the backstory and sets up the events from the first film.

EDIT: Oh, and also you're right since all prequels ARE sequels.