the very repo you pointed to has an explanation of where they got it from:
Unlike its predecessor Google Talk, Hangouts uses a proprietary, non-interoperable protocol. hangups is implemented by reverse-engineering this protocol, which allows it to support features like group messaging that aren't available in clients that connect via XMPP.
basically, the author of that library just reversed those REST api calls. Presumably by using the actual google hangouts, and then looking at what network calls it makes.
I do not believe that google has any documentation on this.
