It’s basically like blocking a whole server, so if threads defederates from mas.to, users on either site can’t talk to eachother

If meta makes special features threads only and once they have a corner on the market start defederating, it could suck other already decently popular services dry (mastodon) since they either move to threads or lose any connections they had on threads. In the end, a cool decentralized thing becomes just another corpo social network.