Having beat my head against Common Lisp, then spent a non-trivial amount of time working in Clojure, I totally agree with his premise.

Learning Lisp requires real, significant use of the language. Eventually you begin to think in the language and that's when profiency begins.

Merely reading about the language is not enough.

I'm sure this applies to learning any new programming language paradigm (as in procedural like C, OO like C++, Java and C#, and functional like CL, Clojure and Erlang) or any natural language for that matter.

[Edit: adding following sidenote.] Reading "On Lisp" was a big part of why I recently bought the $139 Kindle. Reading the "On Lisp" PDF isn't bad with the divice in landscape orientation.