"Suppose you find a stock that you know will increase in value. You go to the bank and say, “hey, can I borrow $90k to buy this stock? I’ll put down a down payment of $10k.” You’ll be laughed right out of there."
And then, later:
"Now suppose you find a house that you think is a good deal for $100k. ... suppose you go into the bank and say “hey, can I borrow $90k to buy this house, and I’ll put down $10k?” If you have decent credit, you’ll get the loan easily."
Leverage, i.e. a multiplication of your investment by borrowing money, is quite frowned upon when investing in stocks[1] but is not frowned upon, in fact positively encouraged, when investing in real estate. This is because "owning a home" has a sentimental value attached to it by policy makers that equity investing does not. It is a weird market distortion caused by bad policy, particularly in the US.
Leverage by retail investors is frowned upon because they can get badly burned and lose a ton of money. Real estate is not as volatile as equities, but as the financial crisis of 2007 showed, large numbers of retail investors going into real estate solely as a speculative investment can lose them huge amounts of money.
In all investing, the size of your return is related to the risk you are taking on. If you can get a 30% return investing in housing, as the author suggests, and only 6-7% investing in mutual funds, that's because housing is 4-5x more risky than a mutual fund. Higher returns are not just sitting around waiting for you to notice how much more money you can make.
So by all means leverage away if you've got capital and want to take on more risk for the chance of higher reward, but don't go into it thinking it's an "almost guaranteed way" to make money, as the author has concluded.
[1] Unless you are a large bank, but that's a whole other story.