Haha, I was the same guy that a few weeks ago was saying that there's no such thing as a "true score". But it clearly does, and private books prove it, as they aim, and succeed, at having the best possible "true score" once out of book, and if it's high enough, you get a "book win" (the game was decided before the engine had to decide what to move.)
So a true score means how likely is that you win the game at the leaf node. There's actually two metrics, how likely is the game to end in draw, and how likely are you to win if the game is decided. Had I been around (with this knowledge) back when computer chess started and it was decided to show scores in centipawns, I'd have pushed for engines to show two scores (the one for winning chances, and another for drawing chances). Without this, engines are prone to play the worse move, where drawing chances skyrocket and it doesn't matter to have more than the opponent, you should be playing the move that keeps drawing chances lower if you have the advantage, even if your chance of winning minus the chance of winning of the opponent is lower than with the other move.
I suspect Robert Houdart figured this out at some point, as after some threshold, Houdini 6 will only show scores between -0.10 and 0.10 if the drawing chances seem too high, and -0.11<>0.11 if someone has decent chances of winning.
The "true score" of a move depends on how strong is your opponent. Against a stronger opponent, you want to increase the true score of the moves that lead to a draw. Against a weaker opponent, you want to increase the "true score" of moves that lead to decided games (0% draw chance is optimal, because it ensures you'll win.) Unfortunately, in practice you have no idea who you're going to face, so the true score falls in the middle.
So that, the "true score" of a move doesn't come from winning chances, drawing chances, or evaluation of position, all that matters is the ELO performance that the move would have against a pool of opponents.
Chess is all about ranking the moves of a position from best to worse, and playing the top one. A true score of a leaf node ends being arbitrary (by itself "0.25" has no meaning, you only put it there to mean some rating performance, so that it's a better one than "0.24" scores and worse than "0.26" scores, making non-leaf nodes to aim for the best.)
For non-leaf nodes the "true score" of a position is the leaf-node's "true score" if both sides play the best performing move, backsolved to the root. What I have is the opening position with 0.03 "true score" for 1.d4 and 0.00 (or worse) for the rest. The Spanish is the best try for 1.e4 but black has nothing better than the Marshall Counter-Attack, and white can't crack it. On the Italian black can force a "true score" that is better for black in the critical variation, no matter what white plays (which to me was a shocker.)
You can recognize critical variations playing against strong opponents that play the same moves you'd have played from the other side, because it means both of you arrived at the same leaf node (for optimal play for both sides), and any deviation from this would lead to worse performance (for this line. Of course the Spanish would have had a better performance than the Italian, so the Italian isn't critical from the opening position, but if you were forced to play it it has a critical line.)
The "David Vs. Goliath" scenario is very useful for detecting leaf nodes that are very good at elo performance. Just let Depth 20 Stockfish play one side of the board, and Depth 30 Stockfish the other. If Depth 20 Stockfish wins you know this variation is very good for that side and you can give it a high "true score."
Otherwise, if, let's say, your mainline leads to a leaf node that gives you a 60 elo advantage, but the opponent is 120 elo stronger, he'd only play 60 elo stronger than you, but they'll still beat you more often than not. I still find surprising that I'm able to gain such an elo advantage as black in the Italian.
What I find so funny is that not only people are ignorant of such things, they actually don't believe you when you tell them and find what you say ridiculous! I never expected becoming the conspiracy theorist of computer chess opening theory!
It is so funny, but I guess it's for the best.
