The theory is that, if someone has an above average IQ, the higher it’s measured the more likely it is that the IQ is the result of lucky circumstances and not a true reflection of the genes he’d be passing along to his offspring. Of course, it’s also possible, but less likely, that the high IQ person might actually be unlucky and he’d actually be passing along better genes than his measured IQ would suggest.
In any event, genetic regression towards the mean would only affect the next generation.
For example, if you gathered together several thousand white gentiles with an average IQ of 130, and they all had children, then according to some theories, the average IQ of their children might be only 124. But assuming there’s no sexual selection for high or low IQ, and they all mate only within the group, all future generations would also have an average IQ of 124.