It's not curated. People used to be asked to repost their content if the admins thought it was interesting, but this no longer occurs. (This turned into a system that could automatically repost content considered interesting; I've stopped posting stories as often, so I'm not sure if it still happens.)

Furthermore, there are protections against vote rings. If, for example, someone votes directly on the URL for a story, or if the referrer is often the same, those votes are discarded.

However, you're right that the algorithm has evolved over time. Visit https://news.ycombinator.com/classic to see the previous algorithm in action.