The bot uses https://instances.social/ to get the amount of users, but it's kind of tricky since it's not actually a good source for this type of info. If you check the website, you can find some Misskey, Pleroma, etc. instances, but not most of them. I couldn't find any kbin instances there, but there were 2 lemmy instances. It also doesn't match up at all with other fediverse stats websites like fedidb and fediverse.observer