You see, someone who is intellectually honest would look up either household or individual inflation adjusted income reports from the fed, which are done by economists who specialize in things like calculating inflation. They would see that the data doesn’t support the argument that income has stagnated and would reevaluate their premise.
This author probably saw that the data didn’t support income stagnation, so they cherry pick “averages”, which are nothing alike. For instance, a house in the 70’s on average is almost half the size of the average house today. Did the average car in the 70’s even include air conditioning, or anti-lock brakes, or air bags? Of course, these types of things are factored into professional economists’ inflation calculations, but since those calculations directly contradict what the author wants to be true, they are discarded and replaced by garbage comparisons of “averages”.