The origin of the Pariah game is a bit shrouded in mystery and unclear since it was only publically announced in 2004 after already years in developement.
Fact is the developement of this game seemed to be a bit of a merger and I spoke to one of the programmers of the game, who were working on the "VehicleGame" which makes the basis for Pariah. It was a completely separate game with vehicles only.
To quote him directly:
I don't know much of the unreal lore, but I can recall that when Pariah began development, the vision was to make a vehicular combat game inspired by Twisted Metal Black/vigilante 8, for the brand-new Xbox live service. Then problems with the physics engine being too cpu heavy, publishers wanting Halo clones, and GTA popularizing get-out-of-the-car designs, we ended up pivoting to First person.
So to summarize, it looks like when it was decided to do a "Halo" clone they took a lot of older stuff and ideas from Unreal projects and tests on Unreal Warfare and merged it all in into Pariah. So I suppose unlike Devastation it didn't start out firmly as Unreal project, but instead took random bits and ideas from experiments/tests and I believe the other division of DE might have been indeed working on an warfare addon at the time, but given that the "vehiclegame" was more of a brainbox's original idea, the other parts are definitely at the very least unreal series inspired.