@mister_nobody: By that logic, so would Heaven. Gotta remember that Hell and Heaven are part of our world (which Iruma is from), just a different plane of existence. Magic and elves are not a good comparison either. If you look up mythology around the medieval era, especially in Britain, Ireland, and Scotland, magic and Elves are a big thing. One could argue that religions focusing on Heaven and Hell are also myths (but that's a can of worms I don't want to open since it will derail the "this manga is (not) an isekai" challenge and potentially piss people off).
Isekai manga tend to go in one or two directions: Reincarnation or being summoned. Both to a completely different planet with a different layout with its own version of Heaven and Hell and typically at open war with the demons. In this case, because humans were able to summon demons (a mythological art dating back to the medieval era again and something many Satanist cults do to this day), it's a safe bet to assume that these demons come from our world's Hell, though both sides of the world consider each other fairy tales.
I will confess: While there is no solid evidence that it is indeed our Hell (I don't recall any of the chapters stating this, but it never said it was a different world entirely either), the evidence I have given is why I firmly believe this is Hell from our world.