The next 10? I don't know, I can't predict the future. And finding out 10 years from now he didn't beat the market for 30 years, is half a lifetime of investing (most of your adult life). Not like you can go back and fix that.
It's 20 years of failing to beat a simple "invest in indexes and forget it". So basically an adult who followed index investing for the last 20 years can say "I have higher returns than Buffet".