Just digging through the HTML and jS source, it appears to be fairly standard HTML with some ReactJS+Firebase on the frontend. It's communicating via Socket.io which implies the backend is running the reference nodejs server:
We use Firebase for Auth and file storage only. Game updates are sent through Socket.io in order to have better control over them, and also Firebase charges per update which is a lot for games.
https://github.com/socketio/socket.io