If you have been here for a year, you must have been there during the huge influx in June, we came a long way since then.
It is true that similar platforms work best with each other: Lemmy with Kbin and Piefed, Mastodon with IceShrimp, Misskey etc.
I personally use mostly two accounts, one on a Lemmy instance (this one) and another with an IceShrimp instance. And I can talk to everyone on those two “sub universes”. But that’s probably expected as platform developers prefer to focus on one aspect (threads or micro blogging) at a time.
Defederation is a marginal event, especially now that Lemmy and Kbin both support instance blocking at the user level.