Is this Hacker News item a sponsored ad? Why doesn't it allow comments?

Another kind of job ad is reserved for YC-funded startups. These appear on the front page, but are not stories: they have no vote arrows, points, or comments. They begin part-way down and fall steadily. Only one is on the front page at a time. The rest are listed at jobs.

https://news.ycombinator.com/newsfaq.html

To answer the question, YC companies can do job ads. It doesn't allow comments since most of them would likely be negative.

This has been the case on HN for nearly a decade, and is likely the most considerate way for YC to get inherent value out of it.

This site pretty much exists to be a free publisher and traffic source for their ads.