It is Blizzard that is too "lazy" to accurately and satisfyingly depict this lore in the in-game skybox, and that is ALL IT IS.
It's not worth the effort required to make it more "accurate" in terms of astronomy and how planets behave, especially given that the world and everything in it is fictional and magical.
They just threw Argus up there and it had to occupy one of the layers. No matter what they picked, someone would find fault with it. But god damn, we're playing WoW, not Kerbal Space Program.