Disclaimer: this is purposefully obtuse.
Other effects in the game which explicitly state they kill you:
Shadows, succubi, massive damage, death saving throws, beholder death ray (notably not even their disintegration ray kills you), power word kill, vampires, mind flayers, night hags, drow inquisitors.
Clearly, if they intended for disintegration to kill you, they'd have said so. Since specific overrides general, and there is no general rule that disintegrated creatures are dead, I rest my case. QED.
We've made their point...a person can survive fireball by making the three death saving throws. Exactly how a fine pile of ash and dust can!
I mean, the guy did roll for death saving throws
Fireball doesn't need to say it kills you. The rules for damage, falling to 0 hit points, and failing death saving throws say that.
Edited to add, damn I refuted him so hard he straight up deleted it.