What they're doing is making it very easy to sign up. You just need to go to your Instagram and click a few buttons and you've created a threads account.
I believe that it's creating the threads account based on your Instagram credentials just like you get signed up on many sites without your Google account
Threads do not do posts. They do twits, or what is called on kbin microblogs. As such Lemmy can not be affected and kbin can only be affected in the microblog section.
The point of ActivityPub should be to become as widespread as possible so as to proliferate the standard and eliminate silos and walled gardens--including Facebook and its ilk. It would be an unambiguously good thing if Meta's (and Tumblr's) move towards interoperability cascades to the other big platforms like Instagram, YouTube, Twitter, and Reddit.
You could follow (or choose not to follow) users on any platform you want, from any platform you want.
Having a single player control most of the market - like meta - means that they will have a lot of sway over how the protocol is developed. This is propably a bad thing since meta har different goals than people currently using the fediverse and also have financial incentives to get people to move over to their platforms instead.
The problem is, walled gardens helps lock-in users. Meta and Twitter et al want lock-in. It only takes the CEO to decide they don't wanna play nicely and boom, back to walled gardens again.