Crazy that Yujiro was so young and achieved so much, but I suppose you have to remember the whole reason behind the Hanma's insane feats comes from the fact that it's loosely stated they descend from a mythological entity. One that became a fallen angel, which spawned the demon capabilities in which each hanma possesses. Jack will probably awaken his demon jaw quite soon, but I believe that may add context as to why they're so insanely strong. Baki-Dou has confirmed the supernatural exists within Baki as well, and further adds new information explaining the inhuman feats. They aren't 100% human.