That is indeed correct. However, not the following mentioned on the page you linked:
If VTK_OPENGL_HAS_OSMESA is ON, and VTK_USE_X is ON, then all the OpenGL
and OSMesa variables should point to the Mesa libraries.
This, AFAIK, is not possible using standard platform provided OSMesa since the that package typically does not provide GLX. If you custom build mesa with OSMesa and GLX support, then it should work.