You do know that TG doesn't use DX11? Or any DirectX for that (It does use OpenGL for its ui though i think??). It doesn't use GPU rendering methods, it uses your CPU to do realistic rendering. DirectX is for gaming, which basically fakes realistic graphics so that you can have your 60fps while getting decent results. Terragen uses the real calculations that take much longer as is (such that you get hours per a frame depending on quality). Hardware tessellation is so models look better when in games as they need to cut down on quality of models when they make them so lower hardware can actually use them etc. But with TG2 you can use your high quality models anyways.
Also everyone has a cpu and thus can do TG to some degree. Only a small percentage actually have DX11 enabled cards (I happen to since I just bought one to upgrade my old stuff). Only reason I have one is that my old card started to die and I game a lot so I see a lot of benefits for DX11
