I'm the one most out of the loop so I rely on someone to correct my misinformation :) but the way I remember it at least from the early days is
SLIME is a minor-mode below lisp-mode that reuses the lisp-mode functionality like navigating by sexp, font-lock, indentation, etc. The dynamic indentation stuff in SLIME, like detecting &body in macros, is used to configure property lists that lisp-mode will see. So really relying on and heavily using lisp-mode.
That could well be wrong and/or out of date though. Just answering due to the novelty of being asked something about that code :)