Actually I don’t think downvotes federate with kbin (I’m not sure about between Lemmy instances?)
Downvotes are not intrinsically part of the ActivityPub standard.
…but yes upvotes federate just fine!

Surely you couldn’t be viewing the post while logged in to upvote if your instance wasn’t federated…

Sometimes it is, say here:
https://fedidb.org/current-events/threadiverse
But you might as well say why isn’t Mastodon included? Kbin is a fundamentally different platform and architecture, even if the two are largely compatible (as is Mastodon, to a slightly lesser extent with Lemmy).
It is perfectly reasonable to look at the growth of Lemmy, think of it like quoting use of one flavour of Linux - Linux uptake overall is also interesting, but not the same.

Congratulations, kbin tells me you have a reputation score of 309!
(Noting, as you did, that kbin will only know about a subset of your activity…)

I think it’s we’ll known Lemmy has a problem with bots signing up (in their millions), hence the warning on that measure in my link above.

If we look at another site:
https://lemmy.fediverse.observer/dailystats
https://kbin.fediverse.observer/dailystats
Basically the same stats.
These seem to suggest 120 posts per day per Lemmy user - I’d agree something is off.

But then try looking at a few users in this thread (which lets note is on a Lemmy instance). From my spot checks about half the comments are from kbin users?

This seems to highlight a common misconception, kbin isn’t really any smaller than Lemmy when we look at active users, in fact it seems it has only just (three days ago) caught up:
https://fedidb.org/current-events/threadiverse
Somehow Lemmy seems to have stronger brand recognition, and people often seem say Lemmy to mean things which include Lemmy and kbin users/platforms.

A saw a post a while back commenting on how many upvotes it was taking to get onto the front page of r/all having dropped, but not sure if there is any way to see stats from before API changes now.