These numbers aren't even close to accurate. The actual position of the US underclass (bottom quintile) is more like the 60-85th percentile of global income. After adjusting for CPI it's clear that poverty in the United States is very real.

https://pseudoerasmus.com/2014/09/29/world-income-distributi...

The US underclass also suffer the social and psychological effects of being the underclass, i.e. weaker social support networks and exclusion from the millieu of political priorities.