For the record I'm the main developer of a XMPP based "social network" project (Salut à Toi), and we are able to communicate natively with others XMPP projects (e.g. Movim) for years (we can share comments like in the video, it's the basis of common standard).
The "many extensions" thing is commonly misunderstood by non XMPP people thinking that it's making software more complicated or hard to maintain. But it's not true: extensions is a strength of XMPP allowing to concentrate on one feature at a time, evolve it, change it if needed, and there is a very good negotiation mechanism. Software are evolving, and it's normal that different clients/servers have different feature, but even with that they can still communicate together. The case is common actually: the websites on your browser can test for implemented features in javascript for instance before activating this or that.
XMPP is not a single technology but a base to support many coherent technologies, for many different use cases.
To go back to this ActivityPub, while I'm a bit annoyed that nobody tried to contact us to join our efforts on XMPP, resulting in yet another standard, at least if it's followed by some platforms, it may simplify the creation of gateways. I've looked at the specs, I don't think that putting "like", and "followers" as the main feature of a social network is a good idea. At first sight, it doesn't seem too difficult to translate to XMPP.