I do like this, every run before this one, even the several where Albus beat the demon king, were runs in which the demon king was simply put not paying attention to him.
And sure the demon king is arrogant, and that is their undoing in several runs, but they're also a piece of shit, their greatest flaw being that they can't bear to see someone else succeed where they would not.
But, as shown here that is also their greatest strength. If someone would, even by happenstance, succeed where they would not, then the demon king will sabotage your efforts with all their might.
Also, from the videogame perspective, albus, you should really have expected the true ending to give the final boss an extra phase.