If everyone's memory is faulty, then history is an unreliable narrator. We are told Adel is an evil tyrant. But perhaps she is a savior who broke from tradition and defeated a Sin-like monster from FF10, akin to Guardian Forces. Or perhaps WEAPONS from FF7.

As a side-effect of it's defeat; global amnesia. As the fog clears, they wouldn't see Adel as a savior. They would see her as the source of destruction holding a smoking gun.