This argument only works if you think of "Lisp" as a single, linearly developing language. It hasn't been that for decades. (That's why they created "Common" Lisp, after all.)
Lisp is a family of languages - within which Scheme and CL are just as syntactically incompatible as CL and Clojure.
CL was created such that it is compatible with a main line of Lisp dialects (those btw. have usually Lisp in their name) -> Lisp 1, Lisp 1.5, MacLisp, ZetaLisp then Common Lisp.
Other Lisps in that main line are Portable Standard Lisp, Le-Lisp, Emacs Lisp, ISLisp, ...
There are quite a few branches with less (Scheme) or more incompatible languages (ML, Dylan, Clojure, ...).
Lisp is a family of languages - within which Scheme and CL are just as syntactically incompatible as CL and Clojure.