The difference is that sci fi is an exclusive genre.Going into outer space doesn't make Dr. Strange a science fiction character. Going to Hell doesn't make Superman a fantasy character. As you say the genre draws from different sources. It's tropes are what separates from the other genres The costumes, codenames and other things associated with, but not necessarily unique to superheroes. A superhero can be a skilled human fighter, an alien, a god or magician.Those who do wouldn't be SF. Case by case basis.
Superman- Alien. So science fiction
Captain Marvel- Powers from the gods via a wizard. Not science fiction.
Iron Man- High tech armor and weapons. Science fiction
Doctor Strange- Master of the Mystic Arts. Not science fiction.
X-Men- Powers from genetic mutation. Science Fiction
Iron Fist- Wielder of a mystical force known as the Iron Fist, Not science fiction.
Green Lantern- Uses high tech alien weapon as a space cop. Science fiction.
And so on
Many of those characters have been in outer space and on space ships. So it is a mixed genre at best,
In fantasy you can have science, harry potter can do a physics equation and hop on a broom the next minute.
The idea that sci fi would allow for magic is a farce.
Its a pretty obvious difference.