The invite-tree they discuss is likely an effective measure. It provides a way of tracking back influxes of bots to responsible pre-existing account(s) and banning them too. And if someone is responsible for inviting many of the pre-existing accounts them too... Making the game of whac-a-mole winnable.

I'm assuming it's equivalent to lobste.rs implementation: https://lobste.rs/about#invitations

The cost of this is adding a ton of friction to joining.