Short answer is no. But I figured maybe a more detailed answer could be nice as well.

They are gradually implementing ActivityPub integration, which would make them a part of the Fediverse. At least that's what they claim to be doing. I'll believe it when they're done.

They are implementing it in several steps. Be aware that this is subject to change - it could be that it has changed already since last time I got up to date, but as nobody seems to have provided a better answer yet, here we go:

• For a while, Threads users outside of Europe can enable ActivityPub from their profile page. If they do so, it'll be possible to follow them from the Fediverse. In Europe this feature is missing, because Zuckerburg is bitter he cannot spy on us as much as he would like to.
• Threads users can see comments and likes from the Fediverse, though it's kept separate from comments made in Threads. So far they cannot respond back to users on the Fediverse, so they cannot participate in the discussion, only see the comments.
• Threads users can follow Fediverse profiles, but not in a particularly meaningful way. It seems they can enable notifications for new posts from users on the fediverse, but the content is not integrated in their Threads stream. They also cannot search for users: They can only interact with Fediverse users that they find somewhere on Threads. So if you follow someone on Threads they can follow you back; if you comment on a Threads post and people like what you're saying, they can follow you from your profile and follow you from there. However, they cannot look up Fediverse users who have not communicated with Threads in any way.
• The Fediblock thingy makes the Fediverse a bit smaller seen from Threads. Not really a big deal for them, but if you're on an instance that blocks Threads you obviously won't have much success communicating with them.

So Threads has a long way to go, but a lot of the technology seems to be in place. They receive comments and content from the Fediverse to Threads, and they shoot content from Threads out into the Fediverse. It's just that they are keeping everything in separate channels over at threads, and they do not yet allow for federation of comments in Threads to allow for back and forth discussions.