So here's the thing: there are quite a few free and open source technologies Google adopted that have remained free and open source... I'm not convinced Google was trying to kill XMPP, and even if it were, the fact that corporations can often successfully kill non proprietary standards isn't a great argument against wanting corporations to adopt those standards.
If content from Threads is available to the Fediverse and vice versa, then there is certainly a risk that 95% of users end up on Threads (e.g., because the sign up experience is better, because it's conceptually simpler for the non tech savvy, and so on) ... And that, by defederating, Threads could force the remaining 5% to move to Threads to continue seeing content.
The issue with that threat is that it has nothing to do with Threads federating. Guess what? The moment Threads launched, it had 5x as many users as Lemmy does. If simply being big and having a lot of content is what allows a platform to scale to the point that it can use the network effect to kill competition, large social media companies have that advantage whether they federate or not.
Worried that Threads will launch features that aren't accessible unless you're on their instance? Well, if they aren't federated... That'll happen anyway.
What Threads joining the Fediverse does, is provide ~1M users on the fediverse access to content from ~5M more users, and means that everyone leaving a platform like Reddit in favor of a platform like Lemmy sees more content. That is 5x more helpful to getting Lemmy to scale than it is for Threads.