To an extent, you're right. But SICP already covers most of the concepts of these languages -- it just chooses to cover them all in Scheme, with a functional bias. I don't see why differing syntax should affect the teaching of the concepts, as long as focus is maintained on one language when it comes to actual programming. That said, this course as it is does teach too broadly to be an effective intro to CS.
It is not just a difference in syntax, it is about the core. While all those programming languages are great in their respective areas, Scheme is directly based on the lambda calculus and as such it defines a minimal amount of primitives/constructions, on top of which you can build almost any other concept from scratch. This is great for a person learning the ropes.
