Buffett lies. He had much better investments. I likes to tell us all stories. The big-finance reality is much more inside-tradingish than he wants us all to believe.
He's had far more income from other investments. But he turned $25 into $2000 in 3 years. That's around 331% profit per year, compounding annually. Given that he likes to invest in relatively mature companies, I can easily believe that no other investment ever showed a greater annualized return.
(Of course this calculation discounts his personal investment of time to run the business.)
I assume he has to invest in established companies just as a practical matter. If you have $50k, you can put it anywhere from the local donut shop to Apple. If you want to invest $50 billion, your options are significantly more limited on what you can do without outright owning the target company.
He makes all his money from leverage. But instead of borrowing from banks or the capital markets whose willingness to roll over debts can be fickle, he borrows via the premia his insurance companies collect.
Away from this passive income has a terrible tax profile--unless you employ leverage (see above).
Warren Buffett claims that the best business that he was ever in was installing pinball machines in barber shops, then splitting the revenue with the barbers. See https://www.cnbc.com/2018/06/19/warren-buffett-bought-a-25-p... for verification.