The server is closed-source, which is all that matters for centralised platforms.

An open-source client would be relevant for something P2P like SSB/Manyverse*, but for all other tech the sourcecode of the server will be your primary concern.

* And even with something like Manyverse, the sourcecode of things commonly used as "de facto servers", like SSB "pubs", ends up still being extremely relevant in practice just because of the UX of using the platform.