It doesn't index the actual article content nor take into account links across sites and content like Google does. Algolia (as self-described) is designed to search for things (like products in a ecommerce store) rather than text with concepts, relations, and entities in a knowledge graph like Google.
Algolia is YC company, so I assume that's the main reason it's being used. But that it does such an awful job with such a simply structured site isn't compelling.
Do you think about any specific improvements? Would you mind sharing with us some non-working queries? We can follow-up here and you can also open issues on https://github.com/algolia/hn-search
Google gives many more results, and a few on the first page seem quite relevant. Most notably: https://news.ycombinator.com/item?id=5531192
But it's the 3rd result in Algolia, behind stories that are both older and with fewer votes.
Let me share that to the team and see whether we can try something.