I think they have to be using OpenGL, as they are "coming back to Mac" and AFAIK directx is no where to be found on Mac. Also with the simultaneous release for both the Mac and the iOS version, I'd think there must be lots of reusable code around OpenGL that'd made the porting to android not an extremely daunting task.
Yes I expressed myself wrong, they must use OpenGL. I assume they have different rendering engines as the paper I linked to mentions an option for software-OpenGL (MESA-like). Also my 'OpenGL is emulated on top of DirectX' is wrong it seems from re-reading that paper, I misremembered.