Sussman is not your average guy, scheme is overall simpler than CL, implementing a compiler is harder than an interpreter.
I would think an interpreter for CL would be easier to write than a compiler. I'm not sure how CL compilation works, but with an interpreter you already have the parse tree in memory which should make scheme's cool features easier to implement.
Really? Are you sure you didn't really just see him copy one that he'd previously written to a chalkboard in about an hour?