No, no one is saying that. That was a sidebar that
@Remathilis started. No one is claimign that all humans shoudl be able to do stuff earth humans can't. What is being said is that very high level fighters should be able to do things that humans can.
So, if you need an in-universe explanation for things, what is the in-universe explanation for high level rogue abilities, which are clearly superhuman and doing things no human can do. No human can dodge fireballs. It's flat out impossible for any human to dodge a fireball. Cannot be done. Yet, a rogue (and to be fair, a monk) can. What's the "in-universe" explanation that is explicitly stated in the Player's Handbook? Or any other book for that matter?
And if it's not explicitly stated, and you're okay with that, why the double standard for fighters?