You understand there is no central authority for this, though, right? Many instances are growing, and account creation is a per-instance concept. The instance in the article is relatively small but it's impacted by activity coming from other, larger instances.
This has happened. Lots of servers, including the ones run by the Mastodon project, are now closed to new users. If you want an account on a Mastodon server then you pretty much need to set one up yourself, or know someone crazy enough to think joining the Holy Order Of Social Plumbers sounds like a good thing.
