I'd say "ah, you mean Jensen's inequality".
Stocks don't follow a zero return random walk. They do, on average, go up in value.
His statement is true. But, the probability of that occurring, and repeatedly is nil. Like saying "if a tsunami hits Nebraska, then a lot of damage will occur". True, but a tsunami won't hit Nebraska. Sort of a false premise.
If I fly in a plane and it crashes, then I will likely die. Should I not fly in planes?
Another viewpoint is that the stock market would not attract $$ if there were better expected alternatives. People accept the risk in stocks because there is an "expected" reward. As one poster notes, by using index funds, you can lower volatility around the expected return.