That's not an official API: http://www.hnsearch.com/about

Quote: "HNSearch was built by the team at ThriftDB to give back to the community and to test the capabilities of the ThriftDB flexible datastore with search built-in."

Interesting API all the same though.

Regardless whether it is official or not, it is pg's preferred api: http://news.ycombinator.com/item?id=4694308

If it were an official API, wouldn't it be associated with HN or Y Combinator rather than an external website?

It is by a YC company, and recommended by pg.