I think the dead horse has been beaten already in this thread, with what the author "should have done" in terms of market research, A/B testing, MVP, etc. No need for me to pile on further there.
But what strikes me is that the author really seems to draw no distinction between personal side projects, done for the purpose of developing new skills that you aren't learning at your day job (e.g. iOS development, Ruby on Rails, etc)... versus entrepreneurial side hustles that you pursue for the dream of financial independence.
I think the the lesson here is, "Pick one". If you're trying to get rich, then that's not the time dive into some completely unfamiliar tech whose learning curve will slow down your velocity. You should be leveraging whatever boring tech you already know and are already most productive in, so you can focus on the marketing research and business side of things. Conversely, if you're trying to dive into an educational side project to learn new skills, then it's rather naive to expect that to be a path to riches. It won't be, and that's okay!