Comment by WhyNotHugo

It's not [attempting to be an open ecosystem]. Their ToS used to forbid using third party clients. I don't think this has changed. They haven't banned anyone for using third party clients (to the best of my knowledge), but they're openly against an open ecosystem.

It's private, centralised and the network is closed (e.g.: non-federated), but the source code is public and open source. I think that for the server implementation they do code dumps every once in a while, rather than continuously keep it public.