ActivityPub has 2 parts: The Server to Server API (S2S), which is how instances communicate and is the backbone of federation. and the Client to Server (C2S) API, which is a way for instances to communicate with an app/website. Unfortunately, mastodon made the decision early on to go the proprietary API route instead of using the C2S for app development. The rest of the microblogging fediverse had to implement the mastoapi so that they could share app support. Lemmy and kbin don't use the mastoapi, which is why they aren't compatible with mastodon apps and they don't even implement the same API so their apps won't be compatible either.

Ideally, lemmy and kbin will migrate to a common API so their apps can be compatible. Even more ideally (and the original goal of the protocol), lemmy and kbin would use the C2S so that they could work with standard AP apps that also work with any compliant AP service.

sure, but I thought ActivityPub is a common protocol they all use to allow interoperability.

It does, but it deals only with content, not with identity federation.

You can access and interact with content with other services that implement ActivityPub (but not all, not every service implements the whole stack).

You can access Kbin posts/communities from your Mastodon account. You can also send messages and see Mastodon content from your Kbin account.