How much is this statistically correct? I agree with the fact that most high-IQ people are not outstanding geniuses, but neither are most non-high-IQ people. This only proves that high IQ alone is not a guarantee for great achievements.

I suspect a statistical error: ignoring a low prior probability that a human has very high IQ. Let me explain it by analogy -- you have 1000 white boxes and 10 black boxes. Probability that a white box contains a diamond is 1%. Probability that a black box contains a diamond is 10%. It is better to choose a black box? Well, let's look at the results: there are 10 white boxes with a diamond and only 1 black box with a diamond... so perhaps choosing a black box is not so great idea; perhaps is there some other mysterious factor that explains why most diamonds end in the white boxes? No, the important factor is that a random box has only 0.01 prior probability of being black, so even the 1:10 ratio is not enough to make the black boxes contain the majority of diamonds.

The higher the IQ, the less people have it, especially for very high values. So even if these people were on average more successful, we would still see more total success achieved by people with not so high IQ.

(Disclaimer: I am not saying that IQ has a monotonous impact on success. I'm just saying that seeing most success achieved by people with not so high IQ does not disprove this hypothesis.)