They are not mutually exclusive. Code identifies a class of language. Specification identifies something that is conveyed through language. Specification can be provided in code.

In fact, while poorly named, what we call testing is actually specification. A test suite specifies for software authors what a program is intended to do and how it is intended to be used. Of course, as a nice side bonus, because the specification is delivered in code the documentation can be validated for truthfulness against an implementation by machine.