Would love to have a chat with you about the issues the people you train have come across with Matrix
I think that he exaggerates the problem. Right now, it's possible for someone to install the Signal client, and then never upgrade it: the Signal servers must have a way, eventually, to refuse service to that client (upon which it will receive a message from the server stating something to the effect of 'your client is too outdated; please update it').
With a federated system, eventually some servers will be so out of date that other servers will refuse service to them. They'll have to send a message to those servers to the effect of 'you are too outdated; please update yourself'; the server can forward that message to its administrator. The clients of such servers might also receive a message stating 'your server is too outdated; please ask your administrator to upgrade or switch servers.'
A similar pattern exists with respect to client updates in a federated network.
Moxie's just wrong on this, I think: it's not significantly worse; the pattern with federation is very similar to the pattern with centralisation; and there are real benefits to federation & decentralisation.
In other words, you are at the mercy of whoever owns the chat platform. To me, this is a bigger privacy issue than the issue with updating servers/clients on a non-federated network.
Matrix/Riot is too new to have those, they have never deprecated a cipher yet.