https://www.forbes.com/real-time-billionaires/#1 was born to a teenage mom (and an alcoholic/deadbeat dad). Since she was still in high school, I assume she was probably in the bottom quintile of income.
#7 was given up by his mother and adopted.
#9 was born in the Soviet Union. It's likely his family's income (in rubles) was in the bottom quintile of the United States at the time. (Sources I've found suggest that the top 1% income cutoff in the SU was around 250 rubles per month, which is less than the $5,000--$7,500 cutoff for the bottom quintile in the US at the time.)
#13 was born in Yemen before his family moved backed to India. One could probably assume that he was in the bottom quintile by US standards, because even today, the top 10% cutoff in India is below the bottom 10% in the United States.
#18 was born in China and I have little doubt that he was far below the bottom quintile cutoff for the United States growing up.
You might think it's a stretch to call Bezos's family "bottom quintile", but saying that billionaires Zhong Shanshan, Xu Jiayin, or Qin Yinglin weren't is hard.