No, it's more like "If fighters cannot be impossible then why is it okay for other non-magical things, like giants, to be impossible?"
Giants are impossively massive because couldn't absorb enough oxygen to survive.
Well, there is no oxygen in most fantasy worlds, including standard D&D. No nitrogen. Just the Element Air. (Dinosaurs could grow to gigantic sizes because the air was so oxygen-rich during that era; the Element Air is similarly different in properties.) The Element Air supports all living creates of all sizes.
Done. Giants are not impossible after all. Fighters mundanely chopping mountains can still be impossible.
Scientifically, dragons would be too massive to fly. We can say that all creatures with wings can mundanely have lift in Element Air.
Done. Dragon flight is not impossible after all. Fighters without wings mundanely leaping 500ft into the air can still be impossible.
But really, who gives a crap? The myths of giants come from an era where nobody knew about oxygen much less the relationship between oxygen density and biology. Centuries ago, people didn't know about aerodynamics and gravity; they thought that anything that has big and strong enough wings can fly.
As a child growing up on fantasy stories, questions about oxygen and gravity was not something I ever cared about in relation to monster size and dragon flight. So suspension of disbelief was not explicitly required.
Sometimes suspension of disbelief works by giving you an answer upfront, sometimes it works by leaving a door open to find a plausible answer if you're curious enough to look for it.
Let's say that I became intensely curious and demanded an explanation. Well, the history of myth and fantasy and D&D's adoption of pseudo-medieval fantasy elemental laws happens to negate my concerns about oxygen and monster size.
It's intuitive to me that the fighter with unexplained martial powers faces a completely different challenge in terms of suspension of disbelief. Asking why dragons fly isn't a concern for me. Asking why a fighter PC (that I have to roleplay or share a story with) has the courage and will to leap 500 ft once a day but doesn't have the courage and will to do something equally impressive does have that problem.
It's like a sort of Schroedinger's Box (or insert better term) where the abilities only work in the fiction as expressed mechanically if you don't ask why BUT if you do ask why the resulting feedback loop from the fiction to the mechanics would alter the mechanics.