I don't believe that these guys were unbeatable masters who were peerless (though the idea of such superhuman prowess is cool) I can imagine that they were supremely skilled, and that Fedor-aura (Fedaura?

) probably made the stories of them that got passed on be told as them being unbeatable.
I do know that My Muay Thai Master's master's master (3 generations of Muay Thai practicioners, going back some seventy years) did actually kill some of his opponents in the ring, though that was the wild west days of Thai boxing, before western style safety regulations were put in place!