It is important to note that the matter of Talos' godhood is still a point of contention, both in how and whether it happened, and it is possible that Septim didn't ascend at all. If he did ascend, it is possible he did so by mantling Lorkhan.
But in Oblivion, one of the main story line quests Martin Septim gives you is to retrieve Tiber Septim's armor, because they need the blood of a divine as a counterpart to another ingredient in the ritual to enter Paradise, which is the blood of a daedra lord.
Well the ritual obviously works as the player is able to enter Paradise, so Tiber Septim's blood must be the blood of a divine. Wouldn't that prove that he did in fact ascend to become a God?