You’re right, a number of them have individually risen decently high on the list, even at the same time, but the family has never dominated it like the original statement said.
This is actually news to me, too, because I was under the impression that there had been a time period where the Waltons’ wealth dominated most others, either before Gates rose to the top, or maybe with him in the top spot but then cleaning up most of the rest of the top 10 list.