The parent comment has been downvoted/flagged to death by users, at which point HN will no longer show it. So no "HN algorithm", just users flagging. You can

The parent comment has been downvoted/flagged to death by users, at which point HN will no longer show it. So no "HN algorithm", just users flagging. You can set the "showdead" flag in your profile and you will see dead posts, including this one (grayed).