Seemed like a dubious claim, so tried to find a source and found this old article:
Did the Forbes 400 Billionaires Really 'Build That'?
https://www.cnbc.com/id/49167533
Trimming a lot:
"United for a Fair Economy breaks down the Forbes list using a baseball analogy. It says
- 35 percent of the list was born in the “batter’s box,” with a lower-middle class or middle-class background...
- 22 percent of the list were born on first base: they came from a comfortable but not rich background and might have received some start-up capital from a family member.
- 11.5 percent were born on second base, the report says. Second base is defined as people who inherited a medium sized company or more than $1 million or got “substantial” start-up capital from a business or family member.
- 7 percent were born on third base, inheriting more than $50 million in wealth or a big company. The report includes Charles Koch and Charles Butt on third base.
- 21 percent were born on home plate, inheriting enough money to make the list."
So less than 35 percent would be coming from poverty, if this study is to be believed. This is for the 400 billionaires list though.