Starting a test where posts from Threads accounts will be available on Mastodon and other services that use the ActivityPub protocol. Making Threads interoperable will give people more choice over how they interact and it will help content reach more people. I'm pretty optimistic about this.
They’re testing read-only access to Thread posts, as far as I can surmise. That’s a far cry from “users on Threads are fully AvtivityPub compliant”. What would really show Meta’s commitment to interoperability would be subscribing to AvtivityPub accounts elsewhere. But of course they want to be the source.
Some or all of the servers you've tried may have them blocked. There's a lot of them that signed the Fedipact promising to do so a while back when they first started talking about doing this.
Or, it may just be that the servers you're searching with/on just haven't properly loaded it yet. I can't currently remember the actual terms for it, because my brain is super-dookie (and I apologize,) so hopefully that makes enough sense 😅
I haven't tried to find it myself, but I've seen other people say they were able to find it from their Mastodon servers, so who knows!
Also it could just be their own bugs or other fuckery, especially since they're only just starting to test it out in the wild. Could be a mix of things ¯\_(ツ)_/¯