There are degrees of offtopicness, of course. In this case I moved the subthread because it was a generic tangent that had little to do with the specific article.

Good on-topic subthreads usually show some sign of contact with the body of the article—not just the title, and certainly not the most generic phrase (in this case "quantum mechanics") that can be abstracted from the title.

But one can always argue particular cases either way and I agree that the counter-argument was stronger for that one.