I don't see that connection. XMPP wasn't big before and it wasn't big after. XMPP is still not big. But it also still exists and is still used.
I think what Google achieved with the usage of XMPP was not some EEE tactics, just as I don't see Threads extinguishing ActivityPub. I think they use open standards as a way to calm regulatory bodies. "Hey look, we have no evil in mind, we use open standards and allow others to play along." Then once they are out of the spotlight, they can slowly defederate again and be a proprietary beast.
Users who leave other fediverse instances to join Threads (or back then Talk) would have also left if they never federated in the first place, simply because they have the bigger network and all their friends are there. The difference is, that they had a relatively brief period of time where you could actually keep communicating with friends without leaving the fediverse.