The study followed users internally as well and monitored their continued behaviour on Reddit. A significant number didn't simply leave, they remained in other

The study followed users internally as well and monitored their continued behaviour on Reddit. A significant number didn't simply leave, they remained in other subreddits and exercised much less of the objectionable behaviour, even if the new subreddit was comparable in viewpoint but stricter in moderation. Part of "it worked" includes the people actually improving their behaviour, rather than just finding a smaller echo chamber.