My point was exactly that the allowed usage you propose, where Emacs Lisp is a Lisp and Clojure, for instance, is not, is quite ridiculous. The disallowed usage, "Which Lisp should I use, Clojure or Common Lisp?" is however, not ridiculous. Your rule excludes a meaningful usage and includes a ridiculous one.

I'm not saying that Common Lisp reduces to s-expressions and syntax extension or even that "lispness" does, only that s-expressions and syntax extension are the features to which the denotation "a lisp" most frequently refers.

Incidentally, I program in Common Lisp for a living and am the author of several libraries in several lisp dialects, which, while not enjoying much use, do demonstrate a working knowledge of the language family. But thanks for imputing my qualifications.