Federation was only addressed insofar as Moxie Marlinspike the developer noted it would be hard to keep the protocol updated if the servers were shared (he compared his tech to Facebook and WhatsApp, and described federated email as "stuck in time").

There's nothing unduly alarmist about the article, but it makes appropriate claims about (among other things) the security limitations that Signal has resigned itself to by remaining unfederated. Read TFA.