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.
It will follow the EEE flow along with their normal anti-competition tactics. First, they embrace: their interest in federation is only to give them the access to content that will make their platform not look empty, allowing them to put their coffers to work on drawing the majority share of users. Then they will extend: they will make sure their platform is compatible with ingesting other server content but others will be unable to federate their content (they will become "incompatible" later, due to "features"). Then they will extinguish competition: they'll cut off what little engagement is left with those (inbound only) federated servers because they no longer need them and the majority of the remaining users will move to their platform because that is where the activity is.
Then Kbin/lemmy will be just like all the other random phpbb instances that no one really uses. Being naive won't make things any less likely, yet there will always be gullible people who argue that "of course they will embrace the technology" and that everything else is just non-sense/wouldn't have worked anyway/blah.
It doesn't take long for the largest servers to have operating costs that they will happily allow Meta to burden in exchange for nearly any concession. The main problem is that, while Kbin/Lemmy is federated, it is federated in a manner that still places content in silos and allows single servers to "own" those spaces. It hasn't really fixed the problem yet, it just spreads the problem out over a few more servers. Until spaces are universal (every server owns a slice of that community, spreading out the community instead of just the users), it will remain ripe for EEE.