TLDR: very few stocks have outsized returns over a relatively short period of time (usually early in the life of the companies). Most stocks (>50%) in the S&P500 (which suggests the DOW would be worse) are just outright bad investments.
But they also say that a lot of stocks are poor investments, so taking "diversify" to the extreme will dilute/eliminate any profits you make. So, in order to provide useful information, they really need to be a lot more specific than just "diversify".