Federating would do more than defederating. The whole argument is that if they stop federating, they'll destroy the fediverse. People will leave activitypub-native instances to go to threads instead because they can't see Threads.
It's actually more important to show people a better way. Granted, if it's toxic, and we just defederate anyway, then it's already not a threat that most of the fediverse would change sides.
The EEE approach is not about whether any particular instance federates or not. Threads would need to create enough value to even make people want to care about them breaking protocol. And more to the point, we are a different culture than back then.
The only threat would be if you're worried Threads would provide substantially better content than here. It'd need to be good enough that a threat of them defederating others is more dangerous. If people made that argument, maybe I could see their point. But so far, I think the bigger threat is the content itself being toxic and garbage. I say let Threads mature a bit and let's see where it stands if and when they do federate (and if we even have to worry about seeing it on Lemmy or Kbin).
And let's be honest, Google didn't kill XMPP just out of EEE. They did so by simply making such a monolithic platform that "everyone" had an account already so why create a separate account for messaging by itself. Pretending Google Talk existed in a vacuum is deceptive at best and objectively wrong at worst. I have a Google account and it's causing me such agonizing pain that it's taking me so long to extract myself out of it. It'll likely take me months or even years to fully remove myself and that's not even counting that I then need to migrate out of Microsoft too.