So ROTT 2013 is primarily a DirectX9 game only? I suppose this is for a range compatibility between multiple systems keep the game running efficiently.

They didn't include DirectX 11 modes for systems that can handle it?

You can switch to OpenGL by editing the ROTTSettings.ini file under your "my documents" folder

...
[Engine.Engine]
RenderDevice=OpenGLDrv.OpenGLRenderDevice
...