The $13.25 is a calculation by Uber, not by the journalist. The journalists re-ran some of the calculations and got slightly lower figure.
"Internal Uber calculations, provided to BuzzFeed News by Uber, based on data spanning more than a million rides and covering thousands of drivers in three major U.S. markets — Denver, Detroit, and Houston — suggest that drivers in each of the three markets overall earned less than an average of $13.25 an hour after expenses."
Assuming Uber are not lying about the $13.25, that would still mean that if you worked 40 hours a week, every week of the year, you'd make $27,560 a year.
Whether this is a good or bad depends on how much ordinary taxi drivers make for a similar amount of work.
Uber has frequently talked about how much a driver's gross income will be as a way of encouraging people to join up, which is a nice bit of marketing, and standard practice for companies like Uber.