From the article: "I put in two, maybe three hours a day on the site, and the checks pour in," he says. "What's not to like?"
My first reaction was that if you spend three hours per day and earn a huge amount of money then why not spend six hours per day and earn twice as much money? He cannot do that because he's maxed his niche. Any additional effort obtains diminishing returns and he cannot scale this success as a business with employees.
Most webmasters who make money operate more than one website (usually by themselves) and if they need more work done, they simply outsource it by hiring writers (if it's a content site), having someone do some PHP work, etc.
I guess this is why most webmasters make a lot more money than the average hacker here. It seems a lot of those on Hacker News fall into the "build one site, one unique idea, pitch it to an investor, form a company for this single website, receive a lot of money and try to monetize it later" idea and waste a lot of time on something that probably won't work, or, if it does, take an incredibly long time to see any returns. In contrast, most of the people making real money don't worry about forming a company (until they start to make enough money to justify it), do everything themselves, and don't try to build the next "killer web app" (most probably have limited programming knowledge) but instead create useful websites that they can monetize via AdSense and CPM networks right away while working on SEO and building content.