"Superhero" is a sloppy term that doesn't help us narrow down the issue -- it's a genre, not a well-defined power level or character type. The very fact that it includes everyone from Superman and Thor to Wolverine to Captain American to Batman and Black Widow, from literal gods and figurative demigods to street-level detectives and vigilantes, should simply demonstrate that it's not really a helpful terminology here.
A character can be a "superhero" without being "superhuman".
(Never mind that comic books are a TERRIBLE source for determining consistent power levels or useful comparative power levels, most superhero comics are written almost entirely under "because the plot said so" logic regarding abilities and power levels.)
The source of the concept of the "Guy at the Gym Fallacy" appears to be here: http://www.giantitp.com/forums/showt...he-Gym-Fallacy Someone correct me if it's older than that. The basic assertion is that gamers, etc, presume that because "Fighters" do things that we can do, they're bound by our limitations, but because "Spellcasters" can do things that we can't do, they're not bound by our limitations. That is, Fighters are bound by what we can't imagine ourselves or anyone we know doing, Mages are bound by what we can imagine them doing. This would be a case of other games putting limits on someone's character because they operate under the fallacy that a Fighter (or similar) cannot be extranormal, and must be bound to "realistic" capability.
The Inverse Guy At The Gym Fallacy, however, is the player who insists that their character is effectively a "normal, if peak ability, human", who has no superhuman powers, no extranormal or supernatural or magic stuff whatsoever... but demands that the character be able to do things that are blatantly impossible for any human in the context of the campaign's setting. Instead of other games putting an unjustified limit on the character, the player of that character is saying "yes, my character is a Guy At The Gym" and then reacting negatively when other players have a perfectly reasonable expectation based on that concept as a self-imposed limitation on the character.