The White Mages of Amdapor did not summon Diabolos.
Diabolos was summoned by the enemies of Amdapor in an attempt to destroy the city. The White Mages sealed Diabolos away within the Sanctum of Dreams. It wasn't even Diabolos that destroyed the city either. The overuse of magic (both white and black) during the War of Magi drained the land of Aether and caused the Elemental of Nihility, Oha-Sok, to become enraged and call forth a mighty rainstorm that flooded Eorzea.
Also, Belah'dia was a 6th Astral Era civilization while Amdapor and Nym both flourished during the 5th Astral Era. We can also say "the enemies" of Amdapor were most certainly not the Nymians, as they were never historically known for practicing Black Magic. The Nymian history, from what we can tell, is also filled with stories of enemies from across the seas attacking, though never conquering, ancient Nym. From this we can conclude that there was a third civilization which flourished during the 5th Astral Era that is woefully unaccounted for in the annals of Eorzean history.
