The problem is not the content of the message that is effectively encrypted, the problem is the metadata that is generated or that can be ascertained through the communication network.

For example imagine that you live in a totalitarian regime, you are A and if you talk to person B you are dead.

In signal we cannot see the content of what A talks with B, but if we know that A has talked with B, the server knows them, you are dead.

In P2P networks if they do not use some kind of routing type tor as session, we can find out if A talks to B as would be the case of Keet, although you can not know the content of the messages because they are encrypted if you can know that they have spoken to each other, you're dead.

With Simplex you can not know that A has spoken to B, simply because there is no such persistent identity and also the sending of messages is rotating between different servers, add also that there is now routing of messages so it is impossible to know where the message comes from. Therefore you can neither know the content of messages nor can you know that A has spoken to B.