Well that’s not quite true. First federation means sharing a lot of metadata: you have to federate so finding users on a federated i hard without sharing metadata. Matrix “leaks” a lot of metadata. Privacy with “what” you say? Yes, but most offer you this. Privacy with who talks to who, where, etc? Nope.

Signal is Open Source and server builds are replicable (you can be sure that they run official on their AWS server), being centralised means that almost no metadata is leaked. But I also dislike the ways OWS and Moxie make their decisions.

I recomend you reading the whitepaper of Session at getsession.org. Session is decentralised and it has all the Signal benefits (Signal protocol, and no metadata). How? Onion routing. As said, read about it I recommend you a lot.