User blog comment:HELO/Le-WHY-athan/@comment-24174486-20120907190710/@comment-3581090-20120907194336

@LilyheartsLiara: Chronologically speaking, in-game, "Leviathan" comes before the end of the game, right? So anything the Catalyst would have added into the narrative would not have been revealed just yet. So the things that the Leviathan says that, to us, echo what we know from the end of the game, would not yet be known to the characters in the game. The DLC provides a retroactive first introduction of the information.

As for your point about the fabled weapon, I can only refer back to my point about its discovery happening off-screen and just sort of being thrown at us as a plot device versus a more organic discovery that we make in the course of playing the game (which is what I would consider the Leviathan).