It depends on the objective of the hackathon. YC's hackathons (I went to both this year) focus on creating a prototype that could be a startup. So the emphasis

It depends on the objective of the hackathon. YC's hackathons (I went to both this year) focus on creating a prototype that could be a startup. So the emphasis isn't so much on the technical programming but more the delivery and business case (kinda like a YC interview I imagine). There are other hackathons I've been to that emphasize deeply diving into a technology or tool. They both have their place. Given that YC is in the business of identifying startups, I don't think it's a surprise that it leans towards an "idea-athon" like you said.