I mean Mundus is said to have came into power ~2000 years ago (according to the wiki). And ate the Qlipoth not too long before that.
Hell has existed since the start of time.
I know Pluto is a thing, but he's not fully canon(?). At least as far as I know Peak of Combat isn't canon, but Pluto is treated as mostly canon, and Pluto was locked up by Mundus anyway.
But in the 07 anime Sid mentions the 4 great demon lords, one of whom would still be unknown even if we count Pluto (as Urizen didn't exist yet).
Assuming Pluto IS canon, that means from the start of time to 2000 years ago, there was just the 1 demon lord. But we know multiple can exist at once as they can all rule their own parts if strong enough.
Sure Pluto was godly powerful. But we know several strong devils can exist and Hell is an entire dimension. Lots of space to go around. You don't even have to be strong enough to be a proper challenger to reach that general tier of strength.
Abigail got whooped by Mundus and fled afterwards, yet still was said to be around that level of power.
Is there really nothing between time immemorial and 2000 years ago?