Do you need to have a solid foundation in lisp to understand it or is it approachable for experienced software developers in other domains?
Just keep in mind that the students in question were at MIT. :-)
The book gets very deep (note some of the things that Paul Graham says in his review, and note who is saying those things). Be prepared to expend a lot of challenging brainwork to get through it, but nope, it doesn't rely on previous knowledge of Scheme. One of the many nice features of Scheme (and lispy languages in general) is there isn't a lot of syntax and so on to learn in the first place.
(procedure argument1 argument2...argumentn) is pretty much it.
It's not the right book for 19-year-olds who want to learn programming. It's the book for 19-year-olds who have been programming since primary school. A complete beginner will struggle terribly, and s/he'd still miss most of what the book is really trying to teach.
Lately, MIT has been investing heavily into obtaining a more cosmopolitan image, so ditching a course aims to scratch the itch of hardcore nerds -- while disappointing -- wasn't unexpected, I guess.