'Ad' doesn't require persuasion to purchase anything. Ads for political candidates (urging you to vote) & many causes (urging you to change other behaviors) are still ads. Often such ads are donated, with no payment for placement.

That said, I think the implicit disclosure via the declaration of affiliation with the site operators – YC – & unique behavior of these posts is sufficient for readers to understand the situation. Even without any label, most people would classify such posts as "job ads".

Whatever harm might accrue to the absolutely densest readers who don't get that seems rather small. Probably less than the harm that other readers might suffer from any crude 'ad' label, which risks leading an even larger group of slightly-less-dense readers to make unwarranted assumptions that such placements are sold for cash via some ad department, or that HN is generally open to paid advertising.

All communication is imperfect, with tradeoffs.

The FAQ literally calls it a “job ad”. Ads don’t necessarily need to drive sales for them to be ads.

It's a "job ad". Someone paid money to have it put in front of your face.