It is actually using the steem json rpc protocol. So if steemit would be down this would still work. Currently I use wss://this.piston.rocks as backend. It is planned that you can change the url to your liking enabling anybody to use their own or other servers. You can read more about the actually implementation here: https://steemit.com/android/@razem-io/building-an-android-app-for-steemit