Turns out, Mastodon interoperability means each Lemmy community is treated as a user on Mastodon. If you tag one of those "user" accounts on Mastodon, it apparently creates a post in the community.
Kbin is very interoperable with Lemmy. At least from the perspective of a Lemmy user, I can interact with Kbin instances as though they're on Lemmy.
But you're right that Mastodon interaction is a bit more awkward. I believe Kbin and Mastodon interact a little more naturally than Lemmy and Mastodon.
Kbin works fine, since it segments its Lemmy-like Threads, and separates them from Mastodon-like Microblogs. You can filter between the two pretty nicely, although Lemmy and Mastodon don't have that kind of filtering on their own just yet, although I believe Mastodon is working on Lists, which are meant to do the same kind of thing, and help avoid clogging feeds.