There is a feature request at kbin’s repository regarding this feature.
I have added feedback to it personally.
There are ideas for users grouping together communities/magazines (from kbin or Lemmy or others) for themselves, and for moderators of magazines to include other communities, or even for admins of instances to group communities together at a site level.
Here is the documentation for server to server delivery in the ActivityPub protocol.
I think you are correct. If there are recipients in the “to”, “cc”, “bto”, “bcc” and “audience” fields, the activity will be delivered more quickly. Mentioning someone in a post is a way to add a recipient.