By shifting moderation responsibilities to the individual mods on each subreddit. Reddit employees do very little moderation on their own - mostly reviewing entire subreddits once they get reported enough times - and banning them entirely if they are unsavoury enough. They basically outsourced it to volunteers in exchange for a platform to have your community on(interestingly, Discord works in the exact same way - discord employees only review servers once they get bad enough and ban them, general reports go to server mods and only to them)