The stats page lists users it knows about, including Federated (see also: the People tab).
Local counts can be seen at: https://kbin.social/nodeinfo/2.0 - currently about 22k.
FediDB uses the nodeinfo for its stats gathering, but has a delay.
It is reporting users it knows about, which includes federated servers. The local stats can be seen at https://fedia.io/nodeinfo/2.0, under users.
I think the Fediverse stuff is just a way of showing how open they are and differentiating themselves from twitter.
Twitter is doing stupid things but normal people find Mastodon confusing. I think Meta may be on to something here that appeals to normal people, and conveniently they can connect to the mastodon community too. Those are also the people who Meta will profit from.
I don’t think it’s a great thing, but in some ways it may benefit the rest of the Mastodon / Fediverse community if normal people are made somewhat more comfortable.
Joining is immediate, where some Lemmy instances require manual approval even now.
The main page comes off as more approachable and familiar. They also have a ton of local communities (or “Magazines”) so people can do a lot even without the Federation. I find the Microblog stuff somewhat confusing, I think because it doesn’t have much of a UI built around it so it is less familiar than Mastodon. It is fairly centralized though, in the sense that there aren’t that many kbin instances out there.