ConvertKit is an example of this.
The founder started by selling books and then made a course on how to sell books, got a pretty big audience around that and then started to build ConvertKit. Eventually he figured out who he wanted to market ConvertKit to and his prior audience of people wanting to sell things online was a great match. Naturally a lot of people who bought his books and courses are a great fit for his email service because the whole premise of his course on selling books is around building an email list. The books ended up being a lead generator for his SAAS app.
That's not to take anything away from Nathan's work. At the end of the day he made a very successful and well deserved business but the specific circumstances around it are a bit different than what this blog post makes it out to be. It was a 10 year journey with multiple good ideas at the right time to get to that point. All of it is documented on Nathan's site. I remember reading some of his posts many years ago.