HN does have job ads for YC startups (described at https://news.ycombinator.com/newsfaq.html). The one on the front page now is https://news.ycombinator.com/item?id=13670103. You can tell it's a job ad because it has no vote arrow and no username. This "reserved slot on the front page for a YC-related ad" approach has been HN's way of giving something back to YC for years. I seem to recall a post by pg announcing it in the early days.
We're diversifying that front-page slot to sometimes show startup launches instead of job ads. Those stories begin with "Launch HN". We figured that would be better both for the YC startups (since just-launched ones aren't ready for job ads yet) and to the HN community (since Launch HNs are hopefully more interesting than job ads).