Given that other people here are saying that only YC funded companies can post a job ad here, maybe it's a "yeah, you knew that already, let's not waste any more time on that stuff" type sentiment. Of course, there are other people saying they didn't know that, so perhaps the main sin they are committing is assuming that the audience is as "in the know" as they are -- ie they are assuming everyone on HN is as aware as they are of how all this ties back to YC, only that's not an accurate assumption.

It's often really tough to figure out how to make sure you are including enough information to inform your audience without including so much as to insult their intelligence. This becomes more complicated as audience size and diversity grows and HN is quite heavily trafficked these days. Perhaps it is better for this company to write the ad so as to not insult the intelligence of the more in-the-know types who realize the ads here are only from YC funded companies than to over-explain. It may be better to offend and alienate the relatively "clueless" than to insult the intelligence of the types of people they hope will actually apply. I mean if someone not in the know concludes they are arrogant pricks and "I wouldn't want to work there", hey, probably no loss. It might even be saving them wasted time (by reducing the number of applications from clueless people). But talking to the highly qualified like they are uninformed 5 year olds who need everything explained, well, there goes your best potential candidates. Good luck filling it now.