Diffuse IMO is still fine for their application if it's done well. What they have now is diffuse too, it's just always the same texture. Using terrain would provide them with some easier texturing and painting solutions I think. The documentation seems to indicate there's at least some form of basic sculpting and painting tools available. I don't know if there's a visibility tool to hide vertices you don't want to see though, which may be a problem.

EDIT: Okay.

About the rendering, damn that's bad. So that's a pro only feature D:
Also yeah just use the trees as reference or whatever.