Casey Newton interviewed Adam Mosseri on the First Year of Threads. He talks about a lot in here, but Adam brings up the fediverse in an answer, and that is of course the thing people like me are most interested in. Here's what he said:

The fediverse is a long-term bet. It isn't driving a lot of Threads growth.It is proving very difficult just to do the basics, particularly from a compliance and a privacy perspective. The standards we're held to are just different, based on our scale. (That's reasonable, that's not a complaint.)

And:

So everything has become at least three, if not six, or seven times more difficult than we thought. But we're committed to it, and we're making progress. We have some announcements coming soon. We just gotta get the basics out. You’ve got to be able to not only post to the fediverse, but to get your replies in the app. Ideally, you should be able to follow accounts from the fediverse in Threads, not just follow Threads accounts on Mastodon servers or clients. So there's still plenty of work to do. But I do believe the world is going to become more open over time. And we should lean into that. And if it takes time, it takes time.

For what it's worth, I think everyone working on Threads has been very consistent in how they talk about federation from the start. It is real, they do believe in doing it right, and the biggest challenge has been in how long it's taking to get it out there.

At this point, Mastodon users (or most any other ActivityPub service) can follow Threads users who have opted into federation, and Threads users get to see likes and replies to those posts in their Threads notifications. To be a fully integrated fediverse citizen, we'll need Threads users to be able to follow Mastodon (etc.) users as well as directly interact with posts from those other services.