I'm not the person you responded to but I would assume that it would look like any other service that uses a different protocol or format on the front-end than the back-end. E.g. a REST service that talks JSON on the front end but speaks SQL to a database. So they have an XMPP backend that uses a translation layer to send a custom AJAX or socket-based protocol to the browser.

Yeah, I was pretty sure that Facebook was just using straight-up XMPP at one point in time. They have been actively trying to drive traffic to their Facebook Messenger app - first by shutting down their XMPP interface, then blocking applications that talk to their web interface, and finally by removing Facebook Messenger functionality from their mobile website.