BlueSky and Threads are looking at interoperable protocols because they a) have engineers at home that think it's cool, and b) see the writing on the wall about upcoming regulation and want to preempt it. This is probably good for other networks already based on interoperability, but there are definitely a ton of open questions.
The article is 100% revisionist history written backwards to justify a knee-jerk conclusion and XMPP is indeed not dead. Or not any deader than anybody else that got washed away by WhatsApp winning the messaging wars over the 2010s.
EDIT: Re-reading my own post, it's too harsh. The article isn't "100%" revisionist history, so much as a biased insider account. The revisionist history is largely coming from both the misattribution of what happened to a deliberate move from Google and the fact that it's being misread and misquoted when people react to it.