You are spot on. The difference between the products/services/values offered by XMPP and AcitivtyPub based fediverse is a very crucial distinction.
XMPP’s value is derived from its connectivity. It is bandwagon effect at work. A single fax machine makes no sense but what about another one? Or another 100 ones? Now you have a positive network externality.
The bulk of the AcitivtyPub based fediverse works very differently. The value is from the content, be it people shitposting or memes or cats. As people who frequent online forums and communities can tell, the majority of members are mere readers. They are content consumers. Content producers are often the minority. The reason why soneone will stick to a particular platform is because of the content and the expectation that more is coming.
The case with XMPP is that Google Talk introduced addons and intricacies that were unique to them. So they could federate with you in full with additional bells and whistles while you were stuck in an eternal catch-up. They presented a better alternative regardless of the eventual defederation. Even if we have some viral clauses as in GPL in open-source software that ensures protocol compatible software to be compliant, we can only do that to a certain extent plus enforcement is always an issue. Who are going to spend the vast sum of money in court to defend the “federation”?
This aside, enforcing federation alone does not ensure decentralization. These zero-marginal-cost fixed-cost-intensive businesses of the internet has a tendency to centralize as serving one more seat costs no penny plus one more seat diluates the fixed cost altogether.