There were two groups that stood out in the book:
1. Professionals who did well and invested wisely. He noted that most professionals who do well, spend all of their income and often even more.
2. Business owners.
Both groups became millionaires through one means: business ownership - through stocks in case 1 and directly through case two.
I know several people who've done very well and it's because of their businesses. A junkyard owner, an owner of an electrician company, an owner of a used tire business are among those who have little or no post-high-school education. They all have far more in the bank than I do.
Note that all of these are all what the blog post calls "unsexy" businesses. They provide a product or service and charge for it.
The first million didn't happen for them in 3 years. But it did happen and for at least two of them (I lost touch with the junk yard owner) the money just keeps coming in.