I think it would be worth looking at a player’s accuracy in terms of their cohort’s standard deviation, given that theory is more or less shared across all players. Even then, the best players now have the best teams and computers, so a lot of Magnus’s accuracy in this game is a credit to Jan Gustafsson et al. I’ve been thinking how you might capture the player’s accuracy out of their prep, that seems a better measure, but even then you’re so often choosing between five +0.0 moves by the middle-game, and you could easily play many totally accurate moves if you didn’t feel like agreeing a draw. I know some have looked at Markov models of a player’s likelihood of a blunder to analyse this instead.

Personally I’ve never felt Magnus enjoyed the modern game with as much opening preparation as we have now. It seems like he’s only in the last few years invested the time in this, instead of relying on his technique to win even from losing positions. I hope AlphaZero proving that fun positional ideas like pawn sacrifices and h4 everywhere reinvigorated him somewhat during his dominant first half of 2019, so there’s still hope the machines haven’t just drained the romance from the game, even if their ideas remain dominant.