I guess it depends on the engine, or maybe that was a rhetorical question. It doesn't take a month, or even a week, though--maybe to master them but not to learn the API. We've worked hard on our little project to design our engine for non-game developers or veterans, it's only a few lines of code to move a square, or draw an image, for example. But I understand the sentiment, especially for a one-time project.
I don't know, they had several well defined constraints and you don't know what a library is doing under the covers without spending some time with it. A proof of concept engine exploration is different than an app with 100,000 simultaneous users. I wonder how many business days they spent on this thing.
They used tools they knew and knew how to scale. Almost always the tool you know is better than the perfect tool. They know redis and websockets, and they made it work. Beats using some engine know one on staff has ever touched?