According to the first page of the letter, they have a pretty good track record.

In 58 years, they outperformed the S&P500 39 times. That is 67% of the time, which according to R comes down to a p value of 0.01:

    binom.test(x=39, n=58, p=.5)    
    => p-value = 0.01193

On the other hand, countless investors started out 58 years ago and 1 in 50 had a performance like this or better by pure chance. So it's hard to say to what extent the performance is an indicator of capability.