It's very "functional" in the "functional programming" sense, but not at all in the sense that 6.003 is about the "functional" descriptions of LTI systems (i.e., that the "signal" output of a "system" is a function of the input, which you can do things like take the Laplace transform of). "Procedural" is the term used in the class's textbook, and the "functions" in Scheme are called "procedures" in the Scheme standard.