I agree, they are not the same. SimpleX is more private than Signal and Matrix for sure. But the protocol still grants a lot of info to the server. 

While Nostr relays also have a lot of info from users, especially if you mix private and public events, a fully private client can make it so that the relay doesn't even know if the computer connecting to them is a user or a proxy. The relay doesn't know if a DM is new or not because date/times are all random. In fact, many GiftWrapped DMs transfers are different encryptions of the same message (or any other private Nostr event), all from random accounts, being broadcasted by bots to generate noise. It fact, that same private client can just transfer directly in P2P if the two phones are online and relays won't even know about it (which is my main use right now)