I agree that the linked question is probably off-topic, but the reason why I think this is the case is that it's a toss up between "questions about software tools" and "primarily opinion-based".
My primary rule of thumb is that if you can publish a paper on it in SIGGRAPH or JCGT, it's on topic in cg.se. So to the extent that modelling is about algorithms, data structures, mathematics, and even physics (e.g. winged-edge vs half-edge, CSG operations on polygon meshes, discrete differential geometry, animation blending and interpolation, solving Hamilton-Jacobi equations on triangular meshes), modelling is on topic.
I don't know where to draw the line, and I suspect we'll know it when we see it. On APIs, I broadly agree with Qix's answer, but I can't help thinking that if questions about the subtleties of OpenGL are on topic, then questions about the subtleties of Collada are also on topic.