No one is using Tor with SimpleX. And even with Tor, the server can perform queue correlations. Ideally, you have to use a different Tor route for each contact/queue in the application to avoid that.

And in large systems like these queue correlation is more than enough to figure out who is who. All the server needs are a good chunck of users outside a Tor network to start mapping everyone out.

Servers can always count and date each message. And the server can lie about the state of the queue, which creates some interesting points of social attacks.