> what is more improbable? that he is flipping a coin 40 times and got head all the time or that billions of people cant see what he sees?

Actually, the probability of getting a head 40 times is 1/2^40, which is 1 in about 1.1 trillion. If we assume there are 1 billions investors overall, the probability that at least one wins 40 times (which is the probability that one or several Warren Buffet could appear by luck in this simple model) is about 0.00091.

So yeah, in this particular phrasing of the question, with this model, the less likely option is to flip a coin and get head 40 times and the more likely option is that there is something else, like talent.