Think about this: YC is a business. YC likes hackers -- hell, YC are hackers. Gaming systems is what hacking is about. Hacker News provides a platform to let their companies get attention. Being front page on HN brings you traffic and tech attention. The HN that you read is the same one potential hires, investors and journalists read.
It's just that if you're in a YC batch or an alum you get posts, especially job posts, boosted.
I wouldn't pay much mind to this talk of mechanisms and gaming systems, sounds like post-hoc reasoning by outsiders.