In the light novel, it's instead implied (and pretty much made official in volume 7) that Diablo didn't evolve from being named by Rimuru. He, like the all the other Primordial/Progenitor Daemons, was already a Daemon King class daemon (which is essentially the Demon Noble rank above Demon Duke). Heck, Moss, one of Testarossa's lieutenant, was already a Daemon Duke prior to being named in the light novel.