It is a different kind of strength and muscle. If you watch American Ninja Warrior, that kind of training they use. Honnold has a hanging wall in his van, what is a finger muscle strength exercise device. He is hanging by his fingers from it. I would say they go for stamina instead of pure strength. Traditionally muscular guys never did well on ANW.
The ways how free soloers can die several times are out of their control. Like a bee attack or a sudden muscle cramp, those things you just can't foresee...