YMOYL was written in a better time, when interest rates were much higher. Not only that, but the book put an undue amount of emphasis on treasury bonds.
I'd recommend you do some further reading on investing. I recommend
-Stocks for the Long Run
-a book by William Bernstein. I read The Investor's Manifesto and it's a solid choice, but maybe someone else would recommend another of his books.
-The Bogleheads Guide to investing is half the people here's bible.
The short of it is this: the riskier an asset class, the higher the payout should be nominally. Stocks have a possible 100% loss rate, so nominally they perform better over the long term. By using indexes you avoid single point failures. Bonds are less risky, and have a cap on the amount of reward. Treasury bonds(especially US for the past 200 years) have almost no risk(the US government failing to pay up? Never -has- happened), so treasury bonds set a pretty effective floor as one of the least -risky- asset classes. However in the current environment it's being manipulated to goose the economy.