"Lack of heart" is relative. When I heard Atlas say that term, I immediately think about Tyson taking loads of punishment from Lennox Lewis and still fighting back/attempting to fight back. His first title defense was against a guy who decisioned him, his first fight that went 12 rounds-that took heart, as well as stamina.
Was he capable of the Chico Corrales comeback against Jose Luis Castillo in their first fight? Maybe not, but only a few are. He was also talking about Mike Tyson hiding from guys in the ghetto when he was little, maybe against kids with knives and guns. That was just a nonsense relationship to boxing. It's an unrealistic standard Atlas was comparing Tyson to.
tuco1808 did a better job explaining this than I did.
edit- by Teddy's definition Wlad K. must be 0-4 in "real fights".