Basically the sequence of events as claimed by the author is that:

XMPP, niche, small circles

Google launches Talk that was XMPP compatible

Millions joined Talk that could coop XMPP in theory

The coop worked only sparingly and was unidirectional, i.e. Talk to XMPP ✅ but XMPP to Talk ❌

Talk sucked up existing XMPP users as it was obviously a better option (bandwagon effect + unidirectional "compatibility" with XMPP)

Talk defederated

This demonstrated exactly the importance of reciprocity. If they play dirty, kick them out asap.

Facebook messenger did the same . You used to be able to talk to fb users via google chat all from pidgin or another xmpp client. They are a hostile actor on the web who have already proventhemseldves untrustworthy. Let's not forget the Snowden docs or Cambridge Analytica