They could even just release the server build to deploy on whatever server you want. No source code. No secrets. Just the client .exe you already have, and the server .exe, and leave it at that. Users themselves could then deploy it on Amazon AWS or just a PC in someone's basement. So the devs wouldn't be required to integrate game hosting onto a client or do anything at all in terms of extra work.
I don't think their servers run on windows.