I bet that "explanation" of the three former heroes as to why the demonlord of light self destructed is a lie.
I bet the true reason is that by using "Relive" on Ash the demonlord lost his own magic mark, became unable to utilize magic and hence as an undead lost the power that kept him "bound" to the world.
But that also means that Ash's condition to be unable to use magic is so hard to "cure" that even such a powerful being as the Demonlord of Light was unable to prevent himself from being afflicted by it.
So telling Ash the truth would be the same as telling him that it is impossible for him to EVER use magic and hence destroying that one single wish and dream Ash has.
Therefore the three former heroes lied to him about why the demonlord died because they could simply not bear to tell Ash the truth.