It is but it's only one-way federating right now. For servers that haven't defederated, they can see threads content, but people from threads can't see fediverse content yet. For now it's basically just a one-way mirror
I don't think I've seen any Threads content on kbin.social nor fedia.io ever. I feel if it was federating I'd see lots of it, no? Since they're so much larger?