No but that's what I'm saying, it ISN'T an answer to his question. They aren't asking why the writer wrote what they wrote, he is asking assuming that world is real, what is the explanation? In this world is it possible for things to just "happen" for literally no reason? If so/not so, what are the implications?
Practical or not, there are people who are treat fictional worlds as models, and those models need to have internal rules and explanations to be satisfying for those people.
Ohhhhh...
Then... Whatever -reason- the Writer wrote into the canon about the thing that is being asked about!
'Cause, y'know. I literally described keeping things "Supernatural" in my Sunset Riders setting by making it the laws of physics of an alternate reality being imposed upon a reality where those are -not- the laws of physics, making it effectively supernatural within the context of the setting.
But, again. Dude running 30ft and swinging a total of 6 times with weapons within 6 seconds at the -end- of the 30ft run. John Henry with a hammer in his hand. Whatever folk-hero baseline the setting's got is the one it's got because that's the one the writer wrote.
For John Henry it's because a normal human being is capable of digging a tunnel through a mountain in a single day if he's good enough, strong enough, and willing to die trying.
For the 5e Fighter it's because they just can. No "Reason" is ever given for them to be capable of performing those superhuman feats. So I guess anyone in the world is capable of it if they try hard enough.
Just like John Henry.
And any distinction of what is or isn't magical or supernatural is going to be determined by the writer but will probably never -really- have an in-narrative explanation of why it is or isn't magic?
Like. Ki. Or Exertion. Or whatever. It isn't -magic- but it's not something -everyone- can do, but I guess anyone -could- do it if they took a level of Monk/Adept/Whatever.
It's weird. But it's all about the writer and the reader's suspension of disbelief, I suppose.