Spock narrates the events that follow AFTER he goes back through time and establishes the alternative timeline. How can he know these things happen? It doesn't make sense.
We can't say that alternative timeline Spock returns to the future either, because he would be going to the future of the alternative timeline, as established by Back to the Future (well except for the Biff part).
I can't wrap my brain around how Spock narrates events that he cannot know about.