Actually?
Where was the Mirror Universes Future Guy?
MU Future Guys absence could be explained by Hoshi murdering young mirror Archer if only Archer was ever going to be any Future Guy in any Universe, and he wasn't alive any more in the present, then it's unlikely that he was alive in the future as well.
Of course, the Empire destroying Romulus could equally explain away the lack of time meddling.
The point being is that future guy had no past in the Mirror Universe, whoever the hell he may or may not have been.