Creation in Planescape is a rather multifaceted story. There are as many creation myths as there are mythologies, and in a campaign filled to the rim with mythologies creation is about as intangible as it gets.
The Yugoloths, Baatezu and Tanar'ri all have their creation myths, but so do their Upper+Planar counterparts. Maybe it comes down to the simple fact that creation is under the laws of the Center+of+the+Multiverse rule; everyone is right, no one is wrong.
Planescape Campaign Setting
On Hallowed Ground