I don’t know ... Duran was at peak form when he said, “No mas.” Sugar Ray Robinson, in 3 decades of boxing never quit like that.
Duran had some sort of a major stomach problem that day. Prior to that he was something like 80-1, the one loss in a pickup fight to the second best lightweight of modern times, Dejesus, who he flattened in two following matches. Duran was a natural lightweight - junior-welter but was simply good enough to beat up people in heavier weight divisions for a number of years after he’d scared everybody close to his own size into the woodwork.