I am not thinking about physics-time, I am thinking about logical-time. If something is in your past, but has no effect on what algorithm you are running on what observations you get, then it might as well be considered as space-like separated from you. If you compute how everything in the universe evaluates, the space-like separated things are the things that can be evaluated either before or after you, since their output does not change yours or vice-versa. If you partially observe a fact, then I want to say you can decompose that fact into the part that you observed and the part that you didn’t, and say that the part you observed is in your past, while the part you didn’t observe is space-like separated from you.
Non central nit: So, you know the things in your past, so there is no need for probability there Doesn’t seem true.
I suppose you mean the fallibility of memory. I think Garrabrant meant it tautologically though (ie, as the definition of “past”).
Pretty confident they meant it that way: