Well ı dont know about that since he clearly seems a demon, dunno if you came to see those reveals. Demon king has a very long fingers and a very tall body, which seems pretty demon. My actual theory is iruma and he is related somehow. As in the first chapters irumas current grandpa takes iruma instead of his family’s debt. Why would an old demon lord would be even giving humans debt, as he doesn’t clearly seem to be affiliated with other humans, it seemed like he approached them to take advantage of their hunger for money and their ignorance for iruma. Of course it would be more interesting if this wasn’t true, but I would like to believe that there is a reason for irumas luck.
I'm not to far ahead, I'm only at the part where Iruma is moved to the demon Kings class with his classmates. So with thst being the case, I don't know if what i say is actually what's gonna happen and a lot of people probably though the same. But anyways, I think thst the demon king was human just like iruma. Since humans have a short lifespan compared to that of an average demon, the demon king ended up dying but no one found his remains. And since the current story takes place hundreds of years after the demon Kings sudden disappearance, the theory does seem plausible. And the demon king probably recieved the somewhat similar power tools to help him from some unknown person.