Why is 21.5% so hard to believe? Sure people can get part time jobs, which is included in the government's calculations, but part time jobs do not pay the bills. Even people with multiple part time jobs can't make it if they have a family to support.
Just because you don't see soup lines doesn't mean people are not struggling. When the person in front of you is buying groceries, you don't know if they are using an EBT card or not.
They have managed to cover up the poverty through entitlement and social programs. The flip side is that they kill the incentive for people to work. It is a double edge sword, because if these people were fully employed, and spending their hard earned cheap money, runaway inflation would collapse the dollar and you would have a civil war on your hands.
According to Trump, 43 million Americans are on food stamps, where the US labor force size is 160 million. That is about 26%.
That rough calculation is a lot closer to 21.5% then 3.9%.