Myth and folklore, for one. For two, Conan the Barbarian goes up against plenty of divine avatars and the like. For three, Charles Atlas Superpowers, particularly in other pulp-era stories like Doc Savage get that concept at least started.
And, finally, as I noted above, we're already talking about a world where even the so-called "mundane" character, even at something like level 10, can just shrug off dragon fire and then decide to chokeslam the oversized lizard that tried to barbecue him. "Mundane" already doesn't mean what it means to us. These people may look superficially human, but they aren't. Limiting them even to only what olympic athletes could achieve IRL is already out of the question--but so many folks demanding feeble martials expect limits even below that.