I think he's anchored his idea of "middle class" to a specific set of possessions, rather than to an economically reasonable definition. Specifically, he seems to be using a 1970s definition of "middle class". He gives a description that's all about having specific things like dishwashers and television. People in 1920 didn't have dishwashers, because dishwashers weren't really a commercially-available thing yet. But they were still middle class in terms of their economic relationships.