I’ve been using and enjoying both. I’ve used Lemmy way more because it has a public API which means mobile apps. Kbin is a bit more to my liking and it’s great on desktop, but it regrettably doesn’t have a public API at the moment. Mastodon is okay as a Twit stand-in but having to brute-force a feed is a learning curve.
Probably uses the Lemmy API to detect which instance you’re logged into or you log in directly when prompted.