What makes it open source is the fact that the parts which matter most are open source. The part that installs on the system is open source, and because of that it can be more easily tweaked and modified to accept other servers. In actuality it can be modified to do so right now, it’s just that there is little reason to do so because the amount of people enthusiastic about snap isn’t very large, as it has many other problems besides just the centralized server infra.