Rephrase that and it sounds nonsensical: "If you can't outperform the stock market, then how can you be sure of anything?" I think Carnegie was just looking for a glib rationalization for his advice to avoid contradicting people whom you want to like you.

What if I am right 9 times out of 10 when I say I am 90% sure of something, but I am never or very rarely more than 50% sure of propositions of the form "This stock's price will go up/down, over a relevant time frame"?

Death by Lob's Theorem to this quote.