Looks like a static site built with Jekyll. Not sure which search plugin they're using here (it may be one of the default Jekyll plugins rather than bespoke code), but I've seen Jekyll sites that use Algolia for search results that are smokin' fast and extremely accurate.
It's kind of neat that they did hand write it in a way. A fun project I'm sure. And they even made it run in a worker to keep things even more quick and responsive.
