Simon Pegg's speculation on that is certainly interesting, but to my mind it ranks below the explanation from Star Trek (2009) itself:
"Nero's very presence has altered the flow of history beginning with the attack on the USS Kelvin, culminating with the events of today, thereby creating an entire new chain of incidents that cannot be anticipated by either party."
"An alternate reality."
Read more: https://www.springfieldspringfield.co.uk/movie_script.php?movie=star-trek
Which makes it quite clear that - at that point - history had changed going forward from the point of divergence only (which is the primary tenet of alt/speculative history).
"Nero's very presence has altered the flow of history beginning with the attack on the USS Kelvin, culminating with the events of today, thereby creating an entire new chain of incidents that cannot be anticipated by either party."
"An alternate reality."
Read more: https://www.springfieldspringfield.co.uk/movie_script.php?movie=star-trek
Which makes it quite clear that - at that point - history had changed going forward from the point of divergence only (which is the primary tenet of alt/speculative history).