May be i the text somewhere, but owning your hmoe outright along with cars would seem to be a huge factor in this.
You have a point there, and could be referring to the spending power of your income. If one is debt free, and I mean completely debt free, making a mere $50,000 is the equivalent of making about $100,000.
Many people are in that interest trap. Mortgage interest, car interest, toys (boat, RV) interest and the worst being that 20% or better credit card interest. A good portion of their income is literally wasted on interest.
We are debt free, we own everything we possess and pay credit cards off monthly. It’s so nice to be able to just buy anything you want to and have the money to pay for it outright.