@pcottle I don't know where to ask about this other than a Threads dev's personal account. I don't seem to be able to see or search for Threads accounts known to have fediverse sharing turned on from my Mastodon server (no, it isn't blocking Threads).
When I try to troubleshoot with curl, I get a 404 for the ActivityPub user:
$ curl https://threads.net/.well-known/webfinger?resource=acct:pcottle@threads.net
{"links":[{"href":"https://threads.net/ap/users/17841400526622740/" ,"rel":"self","type":"application/activity+json"},{"href":"https://www.threads.net/@pcottle" ,"rel":"http://webfinger.net/rel/profile-page" ,"type":"text/html"}],"subject":"acct:pcottle@threads.net"}
$ curl -H "Content-Type: application/activity+json" -H "Accept: application/json" https://threads.net/ap/users/17841400526622740/
{"success":false,"error":"Not found"}