Difference between Kbin and Lemmy is how they show their threads. Lemmy does community self boosting that's why it shows in Mastodon Groups, Kbin does not.
Mastodon working on groups but last I heard about it is in August I think... so nothing yet.
You can manually add lemmy and Kbin threads by posting link into Mastodon search, but again not useful and only when you want to respond not lurking.
I would also want more interactivity with threads as I use my Misskey.io account much more for posting but have to do jump hoops for threads I want to share show up. You can't make threads to Kbin from Mastodon/Misskey as they are sorted to microblogs, while addressing @ to lemmy community will work to make a thread.
All in all we are yet to have good communication between threads and microblogs, my solution for that is set up Mastodon bot that checks RSS feed of my community/magazine and post whenever new thread is made. Afterwards just copy link from bot in mastodon search and you can view/reply.