No mention was made of real estate. While they excluded principle residence as a factor they make no mention of a second home or a rental property.
In many cities real estate has achieved extreme valuations. For example in the Canadian cities of Toronto and Vancouver the average price of house is well over $1 million. Rental stock is at extreme lows. So it is a simple matter to buy a house, two families living upstairs, one in the basement and the three rents put you in a positive cash flow.
Finally, no mention was made of small businesses. Creating a business, and reinvesting the cash back into it has been the hallmark of the "American Dream". From my anecdotal evidence, I know far more wealthy people who come from this lineage, then the world of stocks and money managers.
There is a book written by a self-made billionaire money manager: The 10 Roads to Riches, by Ken Fisher.
Strives to impart the big picture with key details.
Marry well. Really really well... is one of the chapters.