Don't know that this is what the author put in, but what I took out was that he doesn't consciously remember the lives, but there's associative memory. In the Ambien hypothetical, you could imagine all sorts of different flinch reactions developing, without a conscious memory underpinning them.

I thought the author meant, this is how you get "moral progress", and why history becomes less barbaric.

But then, that would require most of the future lives to come at the end of his timeline. Which doesn't really make sense. Shrug.