I was thinking about that the other day. Most fediverse servers won't even qualify to be federated with Threads. Especially single user instances, since they are unlikely to have a privacy policy for themselves and a public stream.

[–] JupiterRowland@sh.itjust.works 1 points 3 days ago

And they can only connect to Threads because Meta doesn't want to go after thousands of private single-user instances, clutter their blocklist with them and check every once in a while if they still exist to keep it from being clutterted too much.

Also, at least on Hubzilla and (streams), it's the single-user instances that are likely to have an actually public pubstream. But not necessarily the federated one that Threads wants.