Well, then let's qualify HN as a mid-sized subreddit with a mostly well-behaved subscriber group and bunch of tricks (such as non-trendy UI) that reduce moderation load, making it possible for a single person to handle it.

Point being, comparing HN to Reddit is a category error; HN is in the same class as subreddits. This is less about HN and more about Reddit itself: Reddit is not a community, it's a network of communities on a common platform. Each subreddit is its own reality.