The majority of his NW comes from investment and savings, not income.
The point of the article is that cutting expenses, saving, investing wisely, and aggressively pursuing pay raises can yield a few million well before retirement age.
These sort of casual dismissals negate the educational value of the post.
1) “How to win friends and influence people”
2) “Never split the difference”
The first one will teach you how to work with people. The second will teach you how to negotiate salaries, raises, and promotions among other things.
I don’t recommend many technical books because engineers can find all of that on the internet and are much more at risk of being bad at people stuff.