Lemmy and kbin are federated so posting on lemmy is just fine and will likely find the same people.
But also if you search for any community in the lemmy search like "!cats@kbin.social" (no quotes) it should let you post in that community like any other lemmy community. Assuming everything is working today on the fediverse.
Direct links in different formats:
- Lemmy style: !cats
- Kbin style: @cats
- Direct url to kbin community via lemmy.world (or your lemmy instance): https://lemmy.world/c/cats@kbin.social
- Direct url to kbin community: https://kbin.social/m/Cats