Because they use ActivityPub, both of them can potentially interact with each other (post, viewing, following, liking, boosting, etc). There’s no way to stop Threads from being able to interact fully with Lemmy if it’s something Meta wanted to implement. Mastodon can already interact with Lemmy directly by following communities, posting to communities, and interact with posts/comments. Lemmy doesn’t go the other way though because it isn’t implemented, but it could be.
Any ActivityPub software/platform/website can interact with any other ActivityPub software/platform/website if it so wants to, because the backend of things like posts/comments/communities/etc are the same across every single platform if they choose to implement them, the difference between the ActivityPub platforms is how they choose to show you information and how you interact with it.