re-read the LN again. In vol. 6 the following is said:
in the case of Diablo, was me naming him. This let him break out of the base demon framework entirely, evolving him from an Arch Demon to a so-called Demon Peer.
obviously in terms of actual power, Diablo was already much, much stronger than his evolutionary stage suggested at the time. IIRC Diablo even mentions at some point that he held back his evolution on purpose because otherwise "the fights would become too boring" or something like that, so he probably COULD have evolved before being named, but just chose not to do so until then.