Actually, via the link, you can see that pg was using Stypi (I too thought it was EtherPad…). The link from the post works: https://www.stypi.com/hacks/13sentences

Edit: Okay, I was wrong. It was originally Etherpad: http://paulgraham.com/13sentences.html

FWIW, here is the HN link http://news.ycombinator.com/item?id=495336