I think you misread the comment. Or at least read it differently than I did. I read that most software engineers who work hard in their 20s and 30s can become millionaires at some point. You seem to have read it that they will become millionaires in their 20s or 30s, which I don't think is what it says. My reading is more that putting hard work in early in the career sets you on a path to pay off debts and start saving and also have the experience to get good jobs later, which allow you to save more. This meshes with my experience of the industry.