I don’t think that’s true. Voyager can search posts.

To see Lemmy content on kbin, just add the full address of the Lemmy instance to the end of the kbin URL. For example, https://kbin.social/m/[email protected] lets you see content from [email protected] on https://kbin.social.

The other way round should also work (ie. to see kbin content on Lemmy), but for some reason it’s not working on major Lemmy instances for me, even though they are federated to kbin.social and other kbin instances.

Lemmy and kbin are two different forum software that can be installed and run on servers. Because both use the ActivityPub protocol, the content between them can be shared. So, a Lemmy user will be able to see content from a server running kbin, using Lemmy.