Spock Prime is the Spock that those who have followed Trek closely over the years know him to be, from TOS and TNG. It is implied so because of his reactions to James Kirk, Montgomery Scott, and his younger self.
In the grand scheme of things, no other universe prior to the events of Star Trek 2009 is being considered....not even the Mirror Universe from TOS.
This is the idea being conveyed to those who follow Trek closely, and it is the general idea of those who know enough about Trek to know just who Spock is.
The only two universes in play here are the Prime (TOS) universe, and the universe established after Nero's arrival.
Why, oh why, would any writer want to have anyone, let alone those who closely follow Trek, wracking their brains out trying to figure out just which Spock they mean by Spock Prime? Folk want to leave the theater having been entertained, not confused.
There is the Star Trek that was, and the Star Trek that is now. The bridge has been formed between two known points. There are no unknowns to consider.
Yes, it might be nice to consider the other possibilities, but that was not the intent with Trek 2009. It's all well and good to think on the possibilities, but not when it has devolved into arguments and general discord.
It would not do to have Spock from universe X be Spock Prime because who knows what the hell went on in that universe. There's not going to be some "Aha!" moment in a future film where all of a sudden, viewers are going to say: "So, Spock Prime was from Universe X, and not the universe established in the original series!" And it sure as hell isn't going to be: "Oh, so Spock Prime is actually Mirror Spock, he just shaved his beard and had an enlightenment period over the years!"
Spock Prime is from that which we know to be the territory of The Original Series.