That wasn't an actual flash forward. It was a literary device, a scene from later in the film. You're meant to think it's a flashback at that point, only to discover later on that it's actually a flash forward. That's one of the things that makes the film great -- it's portrayed as a classic narrative with linear flashbacks until the point at which you realize it's actually a circular narrative, that there are no flashbacks, and that what you thought was a flashback was actually a) a flash forward, and b) a cinematic foreshadowing device.

Tim