Spock's theory was in error. From his point of view it seemed the timeline changed from that point onward. But the reality that we have seen proves that this is a brand new alternate universe.
Or was one all along, even before Nero's arrival.
A lot of people thought that anyway, even before Pegg's recent comments, but either interpretation works (i.e. either the alternate universe always existed, or Nero's arrival created a new one past & present like Pegg said).
The end result is the same.
