I'm not sure the distinction would make enough of a difference, and focusing only on XMPP might be doing yourself a disservice. There was nothing social about Office, but the OP points out how the same strategy worked there as well. Users, overall, tend to go where the other users are. Some people left Digg for Reddit because they were unhappy with Digg, but the vast majority simply followed because it was where the users (therefore activity) went. Reddit wasn't even the best of the many options at that time; what was important was the inflow of users. Once that kicks off, others tend to flock like moths to flame.

As you point out, Reddit was not where you interacted socially, yet it became where you congregated because that was where everyone else was and therefore where the easiest access to content and engagement was. If a Meta product becomes the most popular way to consume ActivityPub content, and therefore becomes the primary Source for that content, independent servers will become barren with just a Meta Thanos-snap of disconnecting their API. They only need to implement Meta-only features that ActivityPub can't interact or compete with, and the largest portion of users will be drawn away from public servers to the "better" experience with more direct activity. (And that's without mentioning their ability to craft better messaging, build an easier on-boarding experience, and put their significant coffers to work on marketing.)

Sure, there will still be ActivityPub platforms in the aftermath. Openoffice/Libreoffice still exists, XMPP clients and servers still exist, there are still plenty of forums and even BBS systems. But, there is a reason why none of those things are the overwhelmingly "popular" option, and the strategy they will employ to make sure that happens is the focus of the article, not so much XMPP.