No, it's not MIX. I believe that MIX will never fly, because it requires support by a participant's server. So unless all server developers will implement it, XMPP users will not be able to use it. Also, it puts too big emphasis on compatibility with old MUC, which is beyond saving.

Our approach does not require anything from a participant's server, and it does not even require a specialized support by a participant client: a reasonable fallback is provided for legacy clients, even if a participant is connected using 2 clients, one of which does support the new standard, and another one does not. It was achieved by clever dancing around how message carbons work and allows to sync clients nicely.