What you're proposing is pretty antithetical to the way the fediverse works. Kbin and Lemmy are two completely different pieces of software that just happen to communicate with each other. There can easily be more (and probably will be in the future) that name their version of a subreddit something else entirely. Heck, Reddit could conceivably add activitypub federation and then you'd literally have subreddits as part of the fediverse.

The entire point is that things are decentralized so the users and developers that make use of the fediverse can do whatever they want with it and so that no single person, organization, or community can enforce their decisions on everyone else.