Well, actuall, OpenGL is not any harder or more complicated compared to D3D or something. In fact, it's easier and more flexible. Plus, it makes ports to almost any platform much simpler.
Anyway, I think especially a small company like Telltale should opt for portability from the start, a good cross-platform engine is not really harder to create, and if you get a Linux or OSX port for free, every copy sold for one of those platforms means cash. If porting is an afterthought, a small company most likely can't afford it.
However, Ryan C. "Icculus" Gordon, well knows Linux and OSX game porter/ coder, OpenAL guru, former member of Loki Games and now at EPIC, always ports one or two games for free (his "pet projects") - if the port is not financialy viable and the original developer agrees to give him the sourcecode (he doesn't release it or does anything with it but porting). He ported Postal 2 and some other games that way, maybe he'd port Telltale's engine, too, if they ask him.