Yes black is only down by one pawn, no big deal but that is when you should rely on theory as much as possible, becuase the game is so close.

If theory is true (that black should attack after he lost the e pawn) then maybe he should play Qh6 on move 18 not 19.

Perhaps Najdorf can answer this. This is what he wrote in 1969 about this game:

"Vain and proud as I was, I thought I had 'made it' as a chessplayer because I had a chance to do battle with an acknowledged chess genius. Under the sway of my emotions, I came out with my prepared opening; Capablanca meanwhile was totally relaxed. Two very pretty women were following his actions as if mesmerized. Without ever taking his eyes off them for long, he built up a commanding initiative. My position went steadily downhill, and the most galling thing for me was the sense of being disdained and made a fool of. In my desperation I tried reminding myself: 'Being beaten by Capablanca isn't a calamity. It's an honor just to have sat opposite him at a chessboard.' Yet I couldn't shake off the feeling of humiliation. 'To hell with chess,' I thought. 'I'm going to devote myself to other things. I'm going to invent a <new> game.' With my head spinning, I set a trap, and Capablanca, true to his usual practice, made his reply without thinking for very long. Luckily for me it was an oversight, and he was soon forced to give up some material. Immediately he offered a draw. I refused."

When Najdorf declined the Cuban's offer with only one minute left on his clock for 17 moves, the word spread like wildfire among the spectators: "That man's mad or a genius!" ("Nowadays we know that he's both!"- Max Euwe )