The more I think about it, the more obvious this an epilogue. It's quite weird, but also very deliberate. It's been consciously structured in script to fit the canonical releases of Shepards adventures as we, the gamer, see them. As in, this is literally the "last Shepard adventure ever". Even though though it's not canonically the final Shepard adventure, as she/he still has the rest of the war to go.
The finality of the conversation between the crew at the very, very end of the DLC consciously observes that there's more of the adventure to go in literal sense, but also that this story is over. It's almost retroactive in its function as an epilogue. We've played the ending already. Even though there's more story to go in the game when you play Citadel, it doesn't really matter, because it is the last thing we may play.
Elaborating on Yanick Roy's World War analogy, this is like having a narrative telling the exploits of a band of soldiers come to an end before the war itself ends, and learning in an epilogue or whatever what happened to everyone. Yeah, there's technically more exposition and development of the world and lore and characters that we know and learn of, but it doesn't matter. What matters is how that narrative arc is structured and how it ends.
So yeah, though Shepard and co go off to fight the great Reaper war and make everybody miserable with the shitful ending, it almost doesn't matter, because for the gamer it's a retrospective. Citadel is the now, and how Citadel ends is how this chapter ends. The rest is, as they say, history.
~fin
