To have 1 MM USD/EUR at the end of your thirties given an average 4 % yearly return on investment, you need to save around 4.000 USD/EUR per month starting at age 25. In Europe, if you earn around 100k EUR per year your free income will be around 5.000-6.000 EUR per month. Saving 4.000 EUR / month is doable with that but you'll have to live very frugally, not traveling much and you better not start a family unless you have a partner who will supplement your income.
And regarding the authors' experience: Yeah, if you happen to be employed in the area that ranks in the top 0.05 % quantile for engineering salaries worldwide you will easily become wealthy, especially if you're lucky enough to be paid in company stock during a stock market bubble.