Defederation is about what an instance allows in, not what an instance allows out. Defederation stops you seeing the defederated instance's content, but it does not stop them seeing your instance's content.
Threads poses some danger to the fediverse, in particular the portion of it centered around microblogging (mostly Mastodon, but also Pleroma, parts of /kbin, etc.), but very little risk to the threadiverse.
The worst thing about the fediverse is all the fondue, but you don't have to eat it.
Emphasis from the original post.
This is a detailed summary, thank you for linking.
I have also read some other POVs here; my fears are not totally allayed and I still think Meta is only engaing with Activity Pub to prevent new, potential competitors arising from it.
I hope the OP is right about it being very little risk to the Threadiverse. The good news is that Threads is focused enitrely on microblogging and not the Threadiverse. Perhaps that means Kbin and Lemmy users will be able to sit on the sidelines and see how it plays out for a bit, idk. Mastodon users will be seeing the most change.