Okay, one more time...
*sigh*
The Star Trek...er...universe(s)...are a reality where time travel into the past is possible. A universe like that is fundamentally different from a universe where time travel is allowed only one way - for example, in our universe, where you can only travel into the future (be it the slow way, like we all do - or the accelerated way, by traveling close to the speed of light and experiencing time dilation.)
In a universe where only one-way time travel is allowed, causality is a one-way street - the past (and present) affect the future, but not the other way around.
In a universe (or multiverse in Trek's case) where two-way time travel is allowed, causality can flow two ways. The past can affect the future...and the future can affect the past. (And everything gets more "wibbley-wobbly-timey-wimey".)
When Nero changed the future of the Trek universe immediately following his arrival, he also would have cause...ripples...that would affect the past. (And the *mechanism* for this backward effect is that same time travel...information in a universe that allows two way time travel...would travel two ways in time. Change the future...your change the past.)
If this wasn't the case, then new Kirk and company could travel back to, say, any of the times the Prime Timeline Kirk and crew were in the past, meet them, and travel with them into the Prime Timeline. Same for traveling back to any of the times Picard, Sisko, or Janeway were in the past, meet up with them, and travel the Prime Timeline with them.
Which would mean that , if events were *exactly 100% the same* for both timelines before Nero showed up, the Alternate Timeline would be full of visitors from a present and a future that wasn't it's own. Not to mention any visitors from it's own timeline! Which would be chaos.
Now, does this mean Enterprise didn't happen...or that someone name Sisko wasn't involved in the Bell Riots or someone from the future helped Zefram Cocrane get the first warp drive ship off the ground..?
No. All of those things could have happened. As both the Mirror Universe from "Mirror, Mirror" and all the other Mirror Universe episodes demonstrates...and as Star Trek '09 also demonstrates, there is a...resonance...between the the various Trek timelines, and people and events seem to...reoccur.
We already know that there was an Archer and an NX class in the JJ'verse/Alternate Universe timeline, because we saw the model and heard Scotty mention beaming Archer's beagle into the Twilight Zone...
In fact, it's a safe bet that before Nero's incursion, things happened pretty much exactly as we saw them on Enterprise. Just because that's how the new writers want it.
But things aren't necessarily 100% the same, IMHO, because I don't think the Kelvin looked exactly how I think it would have looked in the Prime Timeline. There probably was a USS Kelvin, but I'm not entirely convinced that it had a window-viewscreen, or looked exactly like it did in Trek '09. (But that's just my guess.)
What about Kirk and Edith Keeler, and Sisko and the Bell Riots and Picard and Zefram and the first warp flight..?
Any of those things could happen in the new timeline...or not. I'm going to say that they still happened, but this time they happened with the Alternate Timeline's versions of Kirk, Picard, Sisko and Janeway...and maybe they didn't happen exactly as we saw them in the Prime Timeline. And, maybe some of them didn't happen at all. Maybe in the new timeline it was the original, real Gabriel Bell who died in those riots...I don't know. But I do know that there is no sensical way that new Kirk could travel back to the Bell Riots and meet Sisko Prime, and fly back with him to the Prime Timeline's version of the 24th century - because that is not new Kirk's timeline's future.
In fact, if those events *didn't* happen...or happened differently...well right there is one reason why the past in the Alternate Timeline isn't *exactly* like the past in the Prime Timeline. Similar, yes, very similar in fact...but there would be...differences...that would ripple forward...and ripple backwards.
But, based on the way the writers are treating things, it's very much likely that pre-Nero, history of the Alternate Timeline is *basically* the same as it was in the Prime Timeline. But there might be little differences...like what the USS Kelvin looked like, or whatever else differences pop up in the films.
Got it!?