This is the best summary I could come up with:

Joining the fediverse — the decentralized world of social media that includes Mastodon, Pixelfed, and other services that all interoperate through ActivityPub — has been on the Threads team’s to-do list since the very beginning.

Instagram head Adam Mosseri told The Verge in July that he believed decentralizing the platform was key to making it relevant to a new generation of creators.

Skeptics have long held that Threads would never actually federate, even as Zuckerberg, Mosseri, and others at Meta kept promising they would.

For the largest and most centralized social service on the web, suddenly throwing open the gates to other platforms seemed like an unlikely pivot.

This test appears to only cover one small part of a truly federated social network — it doesn’t sound like you’ll be able to post from Mastodon to Threads, for instance, and you can’t move your account between services.

But the test at least reaffirms Meta’s commitment to ActivityPub and to being part of the broader open social web.

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nooooooooon

Honestly I think this is good for the fediverse.

It instantly boosts mastodon and Pixelfed’s reach, which means people won’t dismiss posting there as it never gets seen.

I would never open a Threads account, but it unlocks seeing content from a lot of people I like who ditched Twitter but didn’t understand mastodon.

Yes, this can also be Embrace, Extend Extinguish, but I’m happy for the publicity.

If people don’t like it they can join an instance that defederatrs from Meta and that’s totally fine.