It has not been directx vs opengl really. It's more of a direct3d vs opengl as they are really only renderers and not full stacks to build your game off. That being said, using opengl is smart to do because that is a tough part to get right and the other code(sound, input, networking) should have cross platform libraries or be easy enough to create from the native code.

The relative merits of the 2 APIs have diminished over the years. For high end realtime rendering they merely act as shims for managing various kinds of buffers and activating the shader code for processing those buffers. The rasterisation stage at the end is tiny nowadays. You need separate render paths for different hardware on both D3D and OpenGL. Neither API gives you a fundamental advantage, it's really just a matter of taste and destination OS. I expect the API call layer of their engine to be tiny and interchangeable, think Direct3D 9 vs. 10. (their engine already supports some consoles, too)

Actually they might just use cider (http://www.transgaming.com/business/cider/), which is afaik similar to wine. Half Life 2 and other source engine games seem to run just fine on wine, getting Gold and Platinum ratings (http://appdb.winehq.org/objectManager.php?sClass=version&...), so this looks like the easier route to me.