Funnily enough, Buffett actually calculated this in his essay "The Super Investors of Graham and Doddsville"
Basically, advocates of the Free Market Hypotheses believed that it was impossible for anyone to deliberately, repeatedly generate alpha. The market was rational, and success was probabilistically distributed. With a large enough population, you will get Buffett level returns - therefor Buffett is a fluke.
However, Buffett calculated that there weren't enough investors for his success to be a factor of random distribution, plus there were 2 dozen others who followed a similar strategy who also consistently generate alpha
It's worth noting that Fama has since then walked back on the strong statement of the Efficient Market Hypothesis. I don't know that he ever intended to say beating the market is impossible; rather that beating the market must be impossible, under the rigorously definition of an idealized, "efficient" market.
Unfortunately a lot of proponents of the EMH (especially non-academics) have taken this concept and run with it in ways Fama didn't intend. His position would be more accurately stated as the claim that modern markets are eventually informationally efficient, for any given instance of information asymmetry.
It's a useful model that approximates markets, not an empirical claim about the world.
He definitely never said that beating the market is impossible. He said you can beat the market long term by taking on more risk, but not by market timing and stock selection. Everyone leaves out risk when talking about the EMH.
> The speech and article challenged the idea that equity markets are efficient through a study of nine successful investment funds generating long-term returns above the market index.
Basically, advocates of the Free Market Hypotheses believed that it was impossible for anyone to deliberately, repeatedly generate alpha. The market was rational, and success was probabilistically distributed. With a large enough population, you will get Buffett level returns - therefor Buffett is a fluke.
However, Buffett calculated that there weren't enough investors for his success to be a factor of random distribution, plus there were 2 dozen others who followed a similar strategy who also consistently generate alpha