My guess (not evidence-based, just seems plausible) is that you set up accounts that post stories and comment on each other's. But they also end up posting the 'Great-thanks-for-sharing' comments on genuine writers' pieces, either by accident, because they are blanket bombing (this seems unlikely given the 100 comments per day rule - they have limited ammo), or as cover, so that Medium's systems see them interacting with stories outside their network, and don't flag them up as suspicious. They could just be an enthusiastic and supportive collective publication, whose members also read some stuff from outside.

This cover would also help avoid genuine users reporting them, as they are indirectly benefiting as well from an extra few cents each time the bot reads and engages. The Mafia Don approach - make sure the plebs get some crumbs, and you can steal the loaf and not get reported.

Just a theory.