Yes, the factory fill in the trasnfer case is ATF. I think the main reason they use it at the factory is to reduce the number of lubricants they have to have on hand. ATF is a great hydraulic fluid. It has friction modifiers in it to help the clutches grab correctly. However, neither of those properties help ANYTHING in a transfer case! The trasnfer case needs LUBRICATION above all else. DO NOT use heavy lubricants like gear lube in it! There is a pump inside that lubes the rear bushing. If the oil is too thick, the pump can not move it and the bushing will fail. That is why I use synthetic 5W-30 motor oil. It is about the same viscosity as ATF, but a better lubricant.
Steve Keim