Yeah when I worked at Deloitte Consulting, we had similar training that said something to the effect of "don't put something in an email that wouldn't feel comfortable as a newspaper headline".
It's a common joke, but it's not something you would find literally in the Google-internal official material. They ain't that stupid.
The joke ain't Google specific, I might have picked it up working in finance. If you bing the phrase, you'll find plenty of hits in published articles.