Once up and running, these projects typically require far less time and effort to maintain, and majority of time is spent in improvements so the revenue grows in the future. Or the owner can just sit back, relax, and enjoy the Four Hour Work Week [tm].
As a contrast, consulting gig is good money while you're working. When you stop (vacation, illness, whatever), the money flow stops as well.
It takes quite a bit of level to get it to the point where you can afford to be lazy (passive), which just means spending less time on it, not no time.
That's why income from side project services/products is great in conjunction with consulting (or whatever) income. You can make an extra few thousand a month on a project that you wanted to start in the first place, with some part time number of hours per week.
Even the example in the post is $17k rev with a 6 person team. Although, this guy and his partner are from Romania - it's awesome there and your USD go VERY far.
Best advice I've ever seen on this topic: https://news.ycombinator.com/item?id=13213871
Although my model won't be recurring revenue, so will have to do enough marketing to keep a more consistent churn. Fortunately, I can do some pretty targeted marketing in a service that isn't saturated, and the incumbents haven't kept up well.