Personally, I think superhero is and has been its own genre for quite some time now. While, you can loosely placed it under under the fantasy umbrella, this does not always work. Because as expected you will find pretty much any and all genres represented in the pages of a simple series like, Spiderman for example. If we look at comic books in general this is a common feature. They aren't self limiting, in fact it appears that the more mumbo jumbo a writter piles on, the better. Many of the comic books I read as a childtouhed on elements that were clearly fantasy, yet you also saw sci-fi, as well as supernatural spirituality.
Without a "clearly" defined path it is hard to classify specific works, let alone the medium as a whole. Because that is what it is.
For a genre to be defined it at the very least needs a common unifing theme. And that is something that seems to evade the different comic offerings. Or does it?
If you dig deeper you will see some of the elements central to the superhero universe do provide a rough sketch of what they are about. One aspect, protecting the everyday you and I, is a common element that is found in all of the mainstream properties you will see in shops today. While the idea may seem nobel, people like us are often nothing more than collateral red shirts there to die in en mass.
So when you look, there is a central theme that binds them together forming a genre. It is interesting to look at the different media, how does it sell you on the idea of watching the movies when they are advertised? Rememer they are selling you on the idea so they need a way to tell you waht the film is about. They don't call Guardians of the Galaxy and Batman Vs. Superman a fantasy film, nor do they use sci-fi, rather they make it clear that it is a superhero film.
When you boil it down to its core principles science fiction is about things we think might be possible. On the otherhand, fantasy is about things we know are not. I understand that this is a rather blunt example, at times things are more obtuse than that and it can be hard to tell one from the other sometimes, and of course, there is the issue that science fiction does not always age well, but neither does fantasy. While this can be argued on the level of probable vs. inprobable with fantasy we know the answer. However, there are plenty of classic sci-fi works which were published long before their ideas even got much though, but are today now firmly in the realm of possible, or reality.
Yet, no matter how much they try and sell you the idea that Iron Man is possible, he's rich and smart after all, he is nowhere close to ever becoming a reality. Yet, this does not exclude him from one day being a reality. On the other hand, Doctor Strange is magic and there is little that will ever change that. But yet they have interacted on numerous occasions so what would those stories be categorized as?
It is this ambiguity that leaves many people confused, yet they are two rather clearly defined cases.
What about other offerings in the superhero genre?
Let's take Superman for instance. He is somewhere in between, his backstory, his excuse that "he's an alien" is a very weak explanation for why he has the powers that he has powers.
Does this limit his ability to being categoried into a sci-fi genre? No matter how much it pains me, poorly researched sci-fi is still sci-fi.
And most superhero pieces are just that.
Poorly researched, no matter if the topic is sci-fi, fantasy, or reality. The question is less "is this possible" and more "does the writer think this is possible". Even then, there's soft sci-fi that bends the rules a bit in their favor and hope that the reader does not notice, or at the very least mind too much. One good example of this is Back to the Future. I'm sure the writers knew that a time machine like that is impossible, but it's still sci-fi.
And it is still fun.
And so for those reasons, any form of superhero is never going to fit in a box, unless that box is redefined and named something unique. Thus superhero, as a whole is a genre no matter what subject matter is being touched on.