It’s a reference to this quote:

“John Steinbeck once said that socialism never took root in America because the poor see themselves not as an exploited proletariat but as temporarily embarrassed millionaires.” ― Ronald Wright, A Short History of Progress (source)

Edit: apparently the original John Steinbeck quote was:

“I guess the trouble was that we didn’t have any self-admitted proletarians. Everyone was a temporarily embarrassed capitalist.”