That isn't true. The data comes from the US Census Bureau. If you look at the report he linked, appendix B goes in to details.
The overall poverty rate given is approx 15%.
The overall poverty rate given is approx 15%.
While not a relative poverty measure the regular Census methodology you refer to has all the other weaknesses I mentioned but the biggest is it is based on a rather arbitrary level established in the 1960s then updated based on broad inflation. Not clear how relevant these thresholds are today - may be worse or better.
BTW - Census also has a relative poverty measure, known as the "supplemental poverty measure".