Don't buy into this, this is just marketing. I'm not saying that Signal is acting in bad faith, only that they chose to design a communication silo with themselves at the helm instead of a federation of servers/providers united by the same protocol. Because of that, they own all accounts, and have the monopoly of messages being routing on the network. Of course there is no difficulty for them knowing who's addressing whom, how often, with what kind of payload, by topology. "Sealed senders" and "secure enclave contacts discovery" is just techno babble meaning "trust us, bro. Especially because you have no choice, anyway".