|Diane Sawyer as press aide shortly after Watergate.|
One of the examples that Wallace gives of left-wing media bias is the way Diane Sawyer presented a particular issue. Stewart does not point it out, but Diane Sawyer is a peculiar choice for a source of left-wing bias. She started her career as a Republican press aide and worked as a "literary assistant" (as Wikipedia puts it) to Richard Nixon – after Watergate and even after his resignation.
I don't know if Sawyer has changed her political stance over the past 35 years, but I could not find any indication that she did so in a quick search online. If she remains a Republican supporter, then she is surely a poor example of a source of left-wing bias.