I don't need HN critically, and there are alternatives.
Even if I did, let me clarify something. When I go to amazon.com and start looking for pens, I don't really consider the product listings to be ads at that point, even though each is obviously advertising a product. The listing is the content I am there for. If I go to a company's website to learn about their products, they are clearly advertising their own (first party) products. I'm fine with that.
Note that there is a difference between "an ad" and advertising. Characteristic of ads is that they are not the content you're there for. The real content is independent of "an ad", and you could block or remove ads and the real content would not suffer. Characteristic of ads is that they are not the topic of the site/content/video you're reading/watching (they might relate to the same subject matter, but they are not the topic). Also characteristic of ads is that they're advertising third party products/services, which again have little to do with the actual content.
If you removed a company's products from their own website, or the products from amazon's listing, then you'd be removing the content I'm specifically after. Does not fill my criteria for "an ad."
Now as to how this pertains to HN: HN was specifically created for startup-related links. So if they're advertising startup jobs here, they are very much posting the kind of content users would come here for. That would be actual content, not ads. Also, like a company advertising their own products on their own website, I think it is quite appropriate for YC to advertise companies they've funded here.