Don't really see how. Can you point out how it is a big assumption?
Well for starters, you are dealing with the assumption of absolutes. It is either this way or that way, when the reality of the situation is you don't know anything more or less than anyone else on this forum, save for some developers.
Second it is assumed something will be retocnned to fit the story. Well, for one, game-lore should never be concrete because it leads to discussions such as this entire thread; basically us nerds arguing over the minute details of a fantastical setting. Marrying ourselves to lore and what is said to be true, is not always the truth. For example, how the Prothean's were handled thus far has been great, because it showcased how the study of history is based not always on the evidence given to us, but the assumptions of the evidence at face and contextual value, and how it can always be wrong, and usually is.
Lastly, we know it's taking place some time after the events of Mass Effect 3, we don't know when. The assumption is the move to another galaxy will eliminate the need to talk about the Milky Way, or that it was done during the war as a contingency plan. That is pure speculation,w hich has been passed on as probable due to this whole conversation thus far, despite being an assumption of the details.
So yeah, seems like a big assumption to me on what it all entails.