That was because of the Watchers. When Adrian first appeared in-game, he made a habit of trolling the players, and taunting them with ciphers, puzzles and had a huge hatred for Amber. Being such a dick, and not knowing his name, the players started calling him ‘Diablos’ as an opposite to DiOS. Fred was an inside joke they had going on that kinda stuck. Adrian took a shine to the names and started using them. It wasn’t until closer to the second event that anyone learned his actual name.