It's not just leading public Ashkenazi Jews who propagate this as truth...
The main explanation given for it is selective mating amount the offspring of leading merchants and Talmudic scholars. But for every one of the offspring from this, there are many more offspring in Jewish communities who would have suffered negatively from its effects. Thus average IQ would have been driven down by the latter more than the upward effect of the former.
https://www.bitchute.com/video/DQdwvXKI8MTW/