Every person successfully doing this seems to also be selling something. From the article: "Mr. Jensen also practices an activity that for many FIRE achievers seems to be the new golf: writing a financial advice blog." That blog seems to be mostly affiliate marketing. There's even a comment here already (https://news.ycombinator.com/item?id=17894385) where the author talks about living without a job then posts a link to a book they're selling.
The real story here isn't about retiring early, it's about how to change your career to writing books about retiring early. Writing is still a job.