Everything was done in python. I wrote scripts to query the HN api for the last 1000 comments by PG as well as scrape his site for his essays. Indexed the documents using helpers from nltk.

The front end is flask which queries a mongo store with a little memcached sprinkled in.

Shout out to heroku for all their awesome free addons!

That doesn't answer his question. BuiltWith relies heavily on response headers, which are easily faked, and often not descriptive on the actual tools used. Many use nginx, varnished, even apache to run their applications, regardless of how they were built. It is a nice tool though, I'll admit.