For that matter, I guess The Godfather Part II could be considered a prequel as well. The young Vito scenes aren't flashbacks. Marlon Brando isn't telling a story of when he was a young Robert DeNiro. Michael isn't being told this story, and, as far as we know, isn't necessarily remembering this story. There is no frame story. There is no past nestled in the present. In fact, there is no past or present. There are two independent time periods that are intercut, whose juxtaposition pull out parallels. So, yeah, The Godfather Part II is half-sequel, half-prequel, not sequel with a flashback.