The point isn't which Lisp. It teaches all the concepts common to both Lisps; of the ones from CL, it only misses the idea of a generic function model of OO, and macros. And honestly, once you've had your mind expanded enough by SICP, learning the rest of CL is a snap. (I know; it took me only a couple weeks to become proficient at CL after taking Berkeley's SICP-based course. I wasn't blindingly amazing at macros, but there's On Lisp for that.)

It uses MIT Scheme. There are important differences between CL and Scheme, but IMO those are much less important than their similarities as compared to non-Lisp languages.