The difficulty of modelling fictional heroes in D&D - especially non-wizardly ones - doesn't shed any light on the contrast between fiction and RPGs. It sheds light on idiosyncracies of D&D's mechanics. For instance, the Marvel Heroic RPG does an excellent job of modelling the fictional characters (Marvel superheroes) that players play in that system.
Yeah I'm going to disagree with this as well... I have the Marvel Heroic rpg and what it models is comic books, as in their narrative structure, pacing, idiosyncrasies, inconsistencies, etc. and it does a good job at this... that said IMO it doesn't really model the actual characters very well...
Trying to get my head around this one a little bit Imaro. I absolutely agree with the first part of your statement. A really good encapsulation. MHRP's systemization of comic books into a TTRPG captures those genre tropes beautifully. However, I'm really trying to figure out how the second part of your position works out. It certainly hasn't in my table experience of GMing it (probably...10 sessions at about 2-3 hours apiece?) but I'm struggling to grok your surmise at even the theoretical level. Here is a one quick example of how this systemization has worked out to produce both excellence in genre and fictional character modelling.
Take
this Deadpool datafile that we use for our home game. To properly model Deadpool, he needs to exhibit equal parts:
1) Wile E Coyote - his indestructibleness, his ridiculous ACME toolbox, and his absurdly zany plans that have a tendency to backfire in interesting ways.
2) Bugs Bunny - his 4th wall breaking, relentless wise-cracking, and absurdly zany plans that somehow work out in the end.
3) Super Ninja Mercenary Assassiney-guy.
Current home game features The Avengers + Deadpool vs Thanos. How is the fictional modelling of Deadpool systemitized by MHRP?
* His Milestones reward 4th wall breaking, recklessness and unpredictability.
* His Godlike Stamina, Psychic Stamina, Healing Factor and Immunity make him indestructible.
* His Reckless Limit gives DP's player 1 PP while adding an open descriptor d8 to the Doom Pool, complicating his and the heroes' lives, when he fails a dice pool that includes Bottomless Satchel or Grenades.
* His Unpredictable SFX incentivizes the player to add a DAMN IT DEADPOOL d8 complication on an ally so the Deadpool's player can reroll his action dice pool.
* Obviously his Specialties model his combat acumen while his Affiliations and Distinctions cover the rest of his thematic portfolio.
What does this produce in play? Hilarity, death-wishey, super-powered, over-the-top swashbuckling Looney Toons. Deadpool kicks all kinds of butt. He gets himself in WAY over his head but rallies and recovers in the ninja-est, silliest ways possible (because he is indesctructible with an endless bag of zany tricks). He talks to the audience causing Thor/Cap/Iron Man (one player switches between those three) and Black Widow to go

He drives the team crazy but pulls his weight nonetheless. And he rarely "buddies up" because no one wants to be alone with him because he tends to get that other person in a lot of trouble that is difficult to recover from. And he has a tendency to do solo hijinx to help the team out while the other two buddy up and do normal Avenger-ey stuff.
That is just Deadpool, but Thor, Cap, Iron Man, and Black Widow couldn't possibly manifest more like themselves within the Avenger's genre tropes. I'm not even sure how they could. I suppose if you literally tried your best to ignore all the thematic incentives and synergy embedded into your datafile that brings these characters to life...and somehow engaged in conflicts and approached resolution in ways entirely at odds with the character...
Maybe then?
But then I would be wondering...Ummmm why are you doing this? If you want to be Cyclops, why are you making action declarations, thematic decisions, and RPing him like Deadpool? If you want to be Doctor Strange, why are you playing him like The Thing?
This is feeling somewhat similar to the Paladin vs Fighter conversations we've all had in the past.