I think it's either bots or some spooky spammers that are falsely reporting the number of users. e.g. lemmy.k6qw.com currently claims to have 44k users with no posts...
I believe instances can block other instances entirely, right? Or have I misunderstood that? This would mean none of those bots in there could ever post in our threads here. I'm sure there will be cooperation between the instances on this.