Threads hasn't had federation enabled until now, but you've always been able to interact with Mastodon... sort of. The Lemmy UI doesn't really have a good way of finding Mastodon posts that don't tag a Lemmy community or of following Mastodon users, but if they do tag a community the Mastodon post will show up as a Lemmy post in that community.

You've had some well-meaning but ultimately not quite accurate answers in this thread so just to clarify:

You can follow, post to and interact with Lemmy communities from Mastodon, because they're treated the same way as a "group" on Mastodon in general.

You can NOT follow and interact with Mastodon users from Lemmy, because Mastodon accounts are individual "users" and Lemmy doesn't have the concept of following and interacting with users, only with communities. If Lemmy ever does add a feature to let us follow other users, then in theory following Mastodon users will also become possible.

You can't use mastodon from Lemmy, but you can from some threadiverse software like Kbin.

It doesn't work so well in that direction. Lemmy doesn't have a concept of content that isn't posted to a community. If a Mastodon post tags a Lemmy community, it's available as a normal Lemmy post, but otherwise it doesn't exist.

I didn't fully understand what I was talking about when I replied, and for that I apologize. Now that I know a little bit more, this is basically how it works (I think):

We cannot see posts made directly on Mastodon. However, they can see posts made on Lemmy and even comment on them. We are able to see those comments as normal and without doing anything on our end, but again, that's only as long as they're made under Lemmy posts