Fediverse

That's certainly another theory why these bot accounts don't actually seem to do anything (so far). But it doesn't fit to the picture that lots of naive instance admins seem to ask for help against this.

I am starting to think that it is either some test run or a greyhat that wants to force instance admins to improve registration security before actual bad actors exploit them.

The vast majority of the instances in that screenshot have known jumps from 1~50 users to tens of thousands in less than a day. T

I think that's taking it too far and jumping to conclusions. I cannot think of a single instance of an instance admin inflating their numbers with bot accounts or in any other artificial way, and I've been on fedi before it was called fedi.

This is almost certainly external bad actors taking advantage of captcha-less open signups.