The article is highly misleading, here’s the actual source: https://www.ssa.gov/cgi-bin/netcomp.cgi?year=2014
Those figures are based on forms W-2. So only employee wages are counted. 1099 (independent contractors and investment income) income is ignored in this calculation. And it doesn’t discriminate between those working full time and those working part time (an important distinction made in the official BLS labor statistics [U3-U6]).
The median annual income for anybody who worked on another person’s payroll is around 29k, but the median annual income for an adult in the labor force working full time is significantly higher.