Pedantic point: YC has ads, they are just blend in much better and are delivered in the same medium.

Hiring posts (definitively) and tech posts (maybe) by YC companies. The whole product is one big ad for a venture fund. Its generally well done and unobtrusive. So kudos to them for that it goes relatively unnoticed.

Relevancy is a big point here. HN readers work in tech or are super interested in tech, YC companies do very technical things so hiring posts or launches tend to blend right in for the most part.