Actually that is not correct.
According to the films makers the film employs a different t time travel theory then used in may of the trek stories to have come before.
It used the theroy physicist Hugh Everett postulated Many Worlds Interpretation.This suggested that an atomic particle can and does exist in more than one place at one time.The particle is basically omnipresent.
This means that every decision , every action, or event that has ever taken place in our reality, has resulted in an infinite amount of different outcomes, in an infinite amount of other "universes" or "realities".
This theory basically solves any of the paradox problems associated with the old notion of TIme Travel.
When a traveler arrives in the past, the moment he arrives he has already changed the past . At this point the timeline will branch away from the original timeline, and create an alternate timeline.
THe original timeline will continue, unchanged, and the newly branched timeline will continue on it's own path, with it's history being written for the first time.
Since the time traveler originated from an entirely different timeline, anything he does in the new timeline will not effect the history of the original timeline = Paradox solved.