You had half the story, just the wrong half. HN and YC used to be a single program in Arc (a single thread, even: https://news.ycombinator.com/item?id=21231810). When pg left, the YC side was split off and redone in Rails. In the narrative in my head, this is known as "the great lobotomy". I still wish they hadn't, but oh well.

If you know anyone who would love to program in Arc and wouldn't mind being publicly impaled (<-- the moderation part of the job), please send them my way. It isn't necessary to know Arc, since anyone good can learn it quickly, but it is necessary to like the idea.