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.
基本上,该库的作者只是反转了那些 REST api 调用。大概是通过使用实际的谷歌视频群聊,然后查看它发出的网络调用。