It was either Adolf Hitler or his propaganda minister, Joseph Goebbels, who said that the people will believe any lie, if it is big enough and told often enough, loud enough. Although the Nazis were defeated in World War II, this part of their philosophy survives triumphantly to this day among politicians, and nowhere more so than during election years.
What Sowell points out as the lie is that the gap between rich and poor has widened (because the rich are getting richer, not that the poor are that much poorer). Whether it's a lie or not isn't the issue, but rather the analogy employed to describe the dialectical and political situation. Or, perhaps, I was just reading a parody site of Thomas Sowell's essays (think Poe).