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. These instances also happen to not require a captcha on sign up.
It may very well be that instance owners are innocent as some have really been victims of bot attacks and simply forgot that you could enable captchas for sign-ups, nevertheless I think instance directories like Lemmyverse.net should start disincentivizing anyone from inflating his own instance with tens of thousands of bots in order to get on top of those "leaderboards".
Back of the envelope calculation using approx 10 instances that we know have known a surge of 10k new signups in less than one day, not meant to be exact but that's an accurate order of magnitude.
Fair enough. I think I'm a couple days behind in the state-of-the-bots news. Since asking, I've seen a couple other posts with links to the instances with lots of sign ups and near zero posts. It's not that I didn't believe you (I swear!) I was just curious because I had heard some alternate explanations for previous jumps. Clearly I wasn't paying close enough attention. Not the first time, almost certainly won't be the last... ;-)