I wonder if someone from the Apigee team wrote these, as Google recently acquired Apigee[1], and the guidelines are mostly inline with what Apigee recommends.[2]
[1]https://techcrunch.com/2016/09/08/google-will-acquire-apigee...
[1]https://techcrunch.com/2016/09/08/google-will-acquire-apigee...
Disclaimer: co-author of the design guide.
The Google Style guide is a guide we use (and have been using for years) when designing our own APIs.
(I work on the API team at Google).