We built it as a Firebase app, but we're releasing it as open source and are looking forward to seeing what the community does with it. :-) All of the OT work happens client-side, so it's certainly possible to adapt it to other backends.
Additionally, a lot of the OT code is based on ot.js[1], a fantastic open source OT implementation by Tim Baumann. He has a node.js backend as well as Haskell and Python. Of course then you have to run the server code somewhere and figure out how to persist the documents, etc. :-) Definitely worth checking out though.
1. https://github.com/Operational-Transformation/ot.js/