The big technology crash in the Third Period was the Levianta Catastrophe. The destruction of the Magic Kingdom Levianta made all its Second Period technology salvaged and studied to create their incredibly advanced society. Evidently, its secrets were lost with the nation as no other country seemed to have adopted them in its mainstream culture.
Revolvers exist in Nemesis' time period due to Marlon's invention of the canon and later handheld versions of such firepower during the turn of the 6th century EC. There were surviving (and relatively advanced) revolvers at the Freezis facility beneath Castle Hedgehog following the catastrophe, but they didn't seem to factor into Marlon's independent development of them.
The technology crash mirrors the technology crash in our world (or at least one of several technology crashes we had). Ex, the Romans were advanced enough that they had built engines (even if they didn't think it was practical to use and stuck with slaves) and had working water pipe systems, but after the empire fell it took many centuries before the technology could be reinvented (for the Western world, anyway, which is what Evillious mirrors. I'm not sure what was going on in other parts of the world).
It's a time compressed version of our own world. They lost their advanced technology, then slowly worked their way back up to the modern era again. This isn't a theory so much as the basic premise of Evillious history.