Probably a poorly written article. From prior articles, the 15 cents per mile figure is actually an agreed maximum rate that could be charged (perhaps subject to renegotiation), but of course if demand doesn't support that it would be lowered to whatever the market will support. Then as demand increases the price would, too.
As to being cheaper to fly, not when you add in the costs of a rent-a-car and the gas used at your destination city.
I checked the pdf document. These people really are projecting revenues for 2060 based on $1.06/mile for cars and $4.06/mile for trucks based on some tables, but then, based on another table, they're using $0.125/mile for cars and $0.48/mile for trucks. The latter table with the lower rates is used for financial analysis.