People may also want to read comments on Reddit by one of the document’s authors. Apparently, it’s a work in progress, but they thought people might nevertheless be interested.
I quite liked that they distinguished between idiomatic, normative, and canonical. I have had places enforce quirky stylistic opinions and pretend they are idiomatic for the entire language. https://google.github.io/styleguide/go/index#definitions
If I never hear the word “idiomatic” in style and software discussions again it will be too soon. It’s in the same bucket of facile rhetoric as calling something “modern” (which means: I like it!) or “legacy” (it stinks!).
While greater precision would be nice, it is not in the same bucket as “modern”, about which I agree with your take.
There is an implicit statistical claim being made with “idiomatic” meaning something like “this is how the majority of people do it in the language”. It may also mean “this is how the authors of the standard libraries do it”. Of course you can use it as purely rhetorical device to bolster your argument without any real evidence, but in theory at least claims of “idiomatic” can be backed up by, say, running wide scale code analysis on Github repositories, or examining the standard library.
It’s a perfectly legitimate and useful concept. There are maintenance, onboarding, and other benefits to following common styles.