For the record, at least on their HatThieves.es Pleroma instance, it wasn't a followbot. It was a single person that subscribed to thousands of random people. I know, because I had a short conversation with "ale" about the behavior. He/she believes they are helping federation by subscribing to so many people.
I tried to persuade the person to limit subscriptions to a number he/she could actually follow, but my Spanish isn't good enough.