The one you link considers the upvotes and downvotes as the sample population then constructs the probability that you will upvote it and ranks on that. This allows late posts to take over early posts if they have a better ratio, even if they have significantly less votes, but only if it is "enough" better to make up for its lack of confidence.
This fails to address a few things here:
* It completely ignores people that read the post but didn't vote. There really is no way to get a perfect count of this no matter how much scroll logging, but you could approximate it and then include it in the calculation.
* On HN many people can't downvote.
* As others have pointed HN doesn't necessarily start with a base assumption of all commenters being equal, nor should they.