The previous company I worked at, I hired two people through who's hiring posts. We pretty much tried it out for about six months as we didn't have much luck through the more traditional channels our division's HR department used. So yes, there are real positions being filled through these posts.
That said, just because it's on Hacker News and this is an informal place doesn't mean you don't need to put at least a little effort into your application. I don't think I ever received that many emails in response to a job posting that didn't even contain one complete sentence to go with the resume. I seem to remember that I received multiple emails from candidates that contained fewer than five words. The best one was the one that just contained a random link (presumably to his/her resume, but I didn't need to click on a potential phishing link that badly), no words, nothing else...
The two people who ended up getting hired (one a very junior developer, the other one a mid-level developer) wrote informal cover emails that showed signs of intelligent life at the other end of the keyboard, much like you'd expect from someone trying to apply for a job :).