While there is definitely a correlation between having a low income and taking out subprime loans, it's not 1:1. Having a bad credit score is what pushes people into subprime rates.
Lots of people with higher incomes and bad credit end up with subprime loans, and many people with low incomes qualify for "normal" loans.
What this article basically says is "people with bad credit scores are increasingly delinquent on their high-interest car loans compared to the same time last year."