They won’t come, because they won’t even know about it. A more accurate aphorism would have been “if you build it and tell everyone about it, some of them might come.”
Humans probably don’t want the site to succeed, because they mostly don’t know it exists.
How much SEO happens here on HN? How much do they spend to tell everyone about it? I'm guessing: Not much; maybe zero.
But people come here, anyway.
(That doesn't mean that it's capable of independently sustaining itself, but people do show up.)
You could see every YCombinator investment as a kind of sidelong marketing for HN.
Telling everyone about it is only necessary if you're indistinguishable from 100 different takes on the same thing, and trying to win a shouting match (this includes hurrying to shout the world down before competitors get a chance).
But as the saying goes, you are not in traffic - you are the traffic. The reason you need to shout is because of people like you shouting.