I checked out the repo for the desktop electron app, it talks to a local server. I don't think the backend is open source. Electron is just a wrapper in this case, I would not consider this project open source.
I didn't say the project was open source. The apps are.
You are expecting too much. It's a commercial venture. There is no reason to open source servers.
But not a server, right? Assuming it's based on a server-client architecture.