The Legendarium represents the cultural perspective of the Noldor and the Men who adopted much of their cultural tradition. It's not an objective history, and some things that exist in the world, such as Tom Bombadil and Ungoliant, aren't really accounted for by the Noldor cosmology, even though these beings are known to them.
Similarly, Morgoth isn't just an Elvish version of Satan, he's Satan himself as the Elves understood him - with the implication that Satan is just the same being, viewed through the Abrahamic tradition.
