Back on Jul 5, 2023, Eugen Rochko wrote about Threads and its intention to use ActivtyPub (thus federating with Mastodon).

What is noteworthy about this launch is that Threads intends to become part of the decentralized social web by using the same standard protocol as Mastodon, ActivityPub. There’s been a lot of speculation around what Threads will be and what it means for Mastodon. We’ve put together some of the most common questions and our responses based on what was launched today.

What to know about Threads, Eugen Rochko, blog.joinmastodon.org