Tiber has nothing to do with it and I'm not talking about Ulfric uniting Tamriel under a Nordic flag, I'm talking about independent unilateral support across Tamriel. Its not the first time people have put aside their difference to fight a greater threat, the Empire and Dominion even did it to fight the Maomer in the second era and all of Tamriel did it to put an end to the Sloads after the Thrarisian plauge.
edit: People also need to get over the 'Skyrim for the Nords', even Imperial alligned Nords say it. Its a general dialouge option for nordic combatants across the board.
Well, in that case...Who the [censored] knows. That's Bethesda's call over what side officially wins the conflict, or they leave it vague.
Chances are highly likely the next game alone will be set far and disconnected from the events of Skyrim, in the far or at least distant future with vague and interpretive sense to let the player fill in his own blanks.
I do know one truth. Ulfric is dead. And will always die in any playthrough I feel the need to do the Civil War in. That's my truth, and how it always will be with me. Whether or not he would have led a great uprising against the Thalmor is a moot point, I severed that timeline personally.