There is indeed a caveat in the introduction to the Reference that there may be statements in it that are specific to rustc. However, the authors strive to keep statements about the implementation separate from statements about the language.

The main reason there's not yet an "official" spec is that creating one takes enormous time and money, which are always limited resources. (Note that both C and C++ had no formal standard for over a decade after their initial release.) The Reference is "good enough" to make a formal spec not strictly necessary, and the existence of Ferrocene makes it even less necessary, since anyone who absolutely needs a spec can use Ferrocene.