Jeesh... They just used a little artistic license for the sake of storytelling. Haven't any of you seen one of those movies where the dead person narrates what happened? It's a little cheezy, but a small price to pay to get Leonard Nimoy's voice in the game.
I don't think Spock is dead though. I think at some point he will make an appearance in the game. I could be wrong though. Once he corrected the time line he might have tried to return to his own time and overshot and that's why we are getting the overview.:p