It’s trivial to identify a statistic to support the authors claim. People inherit money all the time, ranging from sums insufficient to cover funeral costs - to enough money to last 1000 generations.

So coming up with “70% lose their wealth” amounts to choosing a threshold on inheritance where people are inheriting good money, but not extreme sums.

It’s easy to burn a 2 million dollar inheritance, and pretty much impossible to lose a billion dollar inheritance.