I suspect it was more that ()-is-nil-is-false-is-0 is entirely visible to the programmer ("see through") rather than an implementation detail hidden by the language.
I still need the occasional reminder myself that nil is () is nil when writing Common Lisp code, lest I do un-idiomatic things like set a slot's initial value to '().
I still need the occasional reminder myself that nil is () is nil when writing Common Lisp code, lest I do un-idiomatic things like set a slot's initial value to '().