Eh, I guess the significance of the word "devil" can vary quite a bit. I mean yes, generally in religions of Earth "devil" is used about the primary force of "evil" or "sin" or whatever negative attribute is assigned to it, but even then I don't think I've ever perceived any "devil" as being at the same level as the god or gods of the affiliated religion, except by people worshiping the devil specifically. Usually the devil is [I]created[/I] by the god(s) in question, and/or is reduced to a subversive role with little to no power beyond what it can exercise through the corruption of mortals. So while "devil" does usually describe a counterpart to a supposedly "good" deity (though I'd say that the "unsullied goodness" of the gods of real-world religions is [I]at least[/I] as questionable as that of the gods in The Prophecy), I would in no way put the two as equals. The only case where a god and devil are equals that I can think of would be in Slavic mythology, and it's not even called the devil in that, so that's ultimately not even applicable. Eh...
And that is even just in regards to religions. Folklore from different parts of the world - I know that this is the case at least in Scandinavian folklore - "devil" is often used about relatively minor disruptive creatures that aren't even necessarily evil, but just mischievous. And if we delve into popular fictional universes - which is perhaps just as relevant, considering that's what the planes is - Dungeons & Dragons makes a point of demons and devils being equal in power, but engaged in eternal war with one another for dominion of (insert name of whatever name that realm/those realms was here), where the main difference between the two was (if I recall correctly) that demons represented "chaotic evil" and devils "lawful evil".

On a side note, because this got me thinking about it, I guess that if one [I]had[/I] to assign that kind of alignment to the demonic factions of The Prophecy, Kreshtaat and the Death Clan would be "chaotic evil", and the Grand Master and the Infernal Empire would be "lawful evil".