Google isn't a monolith. The author is on one team that wanted a way to encourage use of its services. They used an open source project to achieve it. Doesn't seem _that_ crazy to do rather than wait for some other team to build you an API.
The library was a fun side project and has been used by many companies (big and small) over the past few years. Its always nice to see someone make a cool extension on top of gmail.
Here is another one that came out a while ago (uses gmail.js) and on the same theme as this original post:
http://www.slate.com/blogs/xx_factor/2015/12/29/new_chrome_a...
This was just a "let's spend a few hours playing with the Natural Language API" hack day type thing. Robbie's comment perfectly explains the context behind it. Google search pointed me to your library, which fit the need perfectly well, so that's what I used and I really appreciate that you wrote it and gave it permissive license!