I think the true argument is that you can read the Go specification in a slightly extended sitting, whereas (for example), the current working draft for C++ is 1416 pages.
Whenever a difference is found between the two Go compilers, gccgo and gc, the spec is consulted to see which is right. If the spec is not clear on which compiler is right, then the spec is changed so it is.
When the third frontend was written (go/parser and go/types in the standard library), the spec played a similar role. Though far fewer changes were made to the spec, suggesting it is getting pretty good.