Sure -- you're right that the SE engine is definitely only good at specific things (namely, fact/data/science based questions), but it is arguably the best in the world at those particular things.
What is the Quora engine the best in the world at?
I'd look at growth, too. Even the less popular SE sites grow at very respectable rates. How much is Quora growing?
I would imagine most people reading HN have read at least one blog post on his site, if not many more. It's not that amazing he didn't disclose it in my opinion. I agree it could be mentioned in his profile though, just for the curious.
What is the Quora engine the best in the world at?
I'd look at growth, too. Even the less popular SE sites grow at very respectable rates. How much is Quora growing?