idTech5 is full OpenGL. The OpenGL/DX mix was all Tango. They used the engine because it was free for them to do so. But, they had to rewrite almost the entire thing. The renderer, lighting, physics... they retooled almost the entire engine. It's part of the reason why performance is so low in The Evil Within. By rewriting the lighting engine they were forced to remove MegaTextures. As the MegaTexture uses pre-baked lightmaps. And Evil Within uses a fully dynamic lighting engine.