After studying “The Modern Chess Sacrifice” by Shamkovich (almost done), and Sicilian Labyrinth vol. 1 by Polugaevsky, I have come to a decided conclusion. The conclusion is that these guys were super-GMs because of tactics. Crafty has a hard time keeping up during some of these tactical encounters and I could see how they could possibly beat a chess engine with their intuition, understanding and calculation. It’s not all calculated ahead of times. Even Keres admitted once that a combination “looked good”.
Fischer using offbeat ideas and lines to throw his opponents preparation out the window, didn’t simply try to find and play “best” variations. I also think that Fischer could recover from an openings disaster better than any other player. In short, he was highly creative, and I think that this is the part of his game that his opponents couldn’t overcome.