It's a place with bait for software engineers (lots of tech stories and discussion), and YC then gets lots of eyes on job postings for their companies. This is explicitly why it exists.
What is your threshold of what you consider advertisement?
In the same way, speaking with a friend (or anybody) can be seen as "advertising your ideas".
For me, HN is more of an open discussion forum, where you can find lots of critical opinions on the topics, which for me makes it less "advertisement" than lot of other things.
It's a place with bait for software engineers (lots of tech stories and discussion), and YC then gets lots of eyes on job postings for their companies. This is explicitly why it exists.
HN is not ad free, it is an ad.