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.

Edit: Made a few edits for clarification.

[–] Barbarian@sh.itjust.works 3 points 1 year ago* (last edited 1 year ago) (1 children)

Technically, communities/magazines aren't an activitypub thing. The way it's implemented, Mastodon (for example) sees a community as just another user.

A post in that community looks like a "toot" from the community user. Comments in that thread look like replies to the toot.

[–] matt@lemmy.world 3 points 1 year ago (1 children)

They are actually, communities are ActivityPub groups, the issue is that Mastodon does not implement groups (but it is soon!), so what the Lemmy group has to do is boost all the posts/comments so that people can see them in Mastodon.

Once Mastodon gets groups though, the experience should be much better.

[–] Barbarian@sh.itjust.works 2 points 1 year ago

I had no idea! Thanks for the correction

[–] TimewornTraveler@lemmy.world 2 points 1 year ago (1 children)

what's Activity Pub? you're the first person to teach me that name

[–] matt@lemmy.world 11 points 1 year ago

For a non-tech answer, it's basically the "language" used between these websites to make them talk to each other.

If a website uses ActivityPub, it can fetch information (and send information) to other sites that are using ActivityPub in a specific way that's designed for social media.

Another example of this would be SMTP (simple mail transfer protocol) which is what e-mail uses so that different e-mail servers (outlook, yahoo, gmail, etc) can all talk to each other seamlessly.