It's only slightly less accurate to state that Windows is completely described by its binaries.

Both statements depend on your ability to devote time and appropriate expertise to discerning the documentation.

> linux is completely described by its source code.

I would say that linux is defined by the way its source code [in C] runs on machines [of various kinds]. It's also helpful to know some history, especially of unix, for context. And the communities of people that use linux. I think there's more to understanding linux, or even describing it, than the source code only.