That's not exactly what he is saying nor what he means. Both are risky assets. A 100% owned business you deeply understand is still a very risky asset. On its own. I agree with aeden's comment that diversification is one of the main reasons to sell.
The other good reason to sell...you get cash! Sure some of the cash will "probably" go to a diversified stock portfolio but some of it will be spent on cool stuff. You can't use stocks to buy your dream vacation home or donate money to charity for it to be spent on medical research. Cash is also the reason to sell.
Lastly, people die. A business founder must sell the business or give it to someone else at some point in one way or another or it will be done for him or her.