threatening to harvest our conversations without our fucking permission,

Public posts on social media are well public. There's literally nothing that stops them from reading your posts without federation, and federation does very little to change that in a meaningful way.

or using Microsoft's tactic of "embrace and extend" to destroy the competition

By what, adding features to Threads that make users prefer it over other ActivityPub implementations? The worst case here is that Meta federates, then does something that breaks federation and their users don't jump ship to another instance but stick with the Meta product?

Anyone can implement ActivityPub. We have no problem with that.

So long as they aren't a giant corp you personally see as especially evil, then if they implement ActivityPub they need to be blocked as globally as possible?