Meta announced that users on Threads will be able to see fediverse replies on other posts besides their own. In addition, posts that originated through the Threads API, like those created via third-party apps and scheduling services, will now be syndicated to the fediverse.
Because it is.
Lemmy has 2 main devs.
Piefed is developed mostly by a single person.
Mbin has a few more, but not much.
There’s only so much development teams this small can do.
To come back to your question above: you can use https://fedia.io/ to connect to both Lemmy communities and Mastodon.
It uses Mbin.
I think you can import your subscriptions from Lemmy (not 100% sure)