I have no information to discount K.D. Williamson's information. This $5 (average) would be what someone has after subtracting debt from assets. By mentally "calculating" the other statistics - size of college loans, their investment resistance and being viewed as high risk loans and vulnerable to scams - in the piece, this is not outside the realm of reality.
Your points are well taken. And cultural differences (much due to associations - staying on the liberal plantation would be one way to say it) are pointed out by Williamson as having great import in a group's success in acquiring wealth, mobility and generational movement up the socioeconomic ladder.
Yes, the truth that people don’t want to recognize is that not all cultures are even remotely equivalent in objective value as is easily measurable by objective results.
Black culture, as practiced by so many people of that race, is a disaster. It does not encourage the values which contribute to success. However in our nonsensical multicultural world, it is celebrated as equal in value to other cultures which as a contrast actually produce results (like Asian immigrant culture for a shining example).