My guess is that the Kelvin timeline was one of those already-existing universes - like with Defiant and the Mirror, they traveled back while shifting over. Alternatively, being within the realm of the event that is enacting the rewrite, there may have been a delay or a protection involved by being within the events gravity/EM field/something sci fi. Until the Kelvin-verse gives me a definitive ending/reset button restoring the Prime, I'm going to go with "always existing alternate." This is all my personal choices and perspectives, of course. To each our own.