How much of modern chess is strategy vs brute analysis/skill?
I think it's a lot of both.
There's a lot of theory for openings and endings, whereas the middle part of the game I think tends to be more about brute analysis.
With the openings in particular, there's a lot of theories on what moves to make first, and how to respond.
What I've heard is that for grandmaster level, the difference of having an extra pawn is often enough to win a game.
Is knowing all these past games partially why he's so good, or is it a byproduct of him being so good?
Based on what I've heard, it's a bit of both, but probably moreso the latter.
Some have talked about how he would often intentionally have a strategy of making unusual sequences of moves, so that he could beat them on the analysis.
It doesn't seem to be very unusual to be able to memorize games like that for International Masters/Grand Masters, but it sounds like he manages to do that better than anyone else.