I don't really think it was a flashback.
When you have about 12 volumes of backstory, thats more like displaced time. If you wanted to, you could say that the beginning of the manga was a flash-forward.
I thought it was kind of obvious early on that Miura didn't really know where the story was going, but it pulled together quite well.
Kind of raging at the fact that considering the manga has been around since before I was born, its still only a third done and the characters just really got started on a tangent.