They could discard the endings, or make the DLC about a different character entirely. I'm honestly not interested in going through the same story twice, let's find the cure for V: part 2. I really doubt they would go that route either (because it would be against their own words of not "withholding story").
Plus, If we consider that every ending is happening outside of NC, and assume that the DLC would be based around NC then, the DLC would start with "let's ignore what happened in those endings, we are going back to NC", which doesn't make sense.