He is not arguing that it is not. He is arguing the opposite of "in a rising tide all boats rise". He is arguing that the tide is falling.
Based on what evidence? Standard of living has been rising in this country.
Certain people on the left use a so-called "gap" as evidence. Let me give you a scenario. Your neighbor Joe, is slightly wealthier than you. Say his net worth is 3% greater than yours. Suppose that I want to purchase some asset from each of you. I will pay Joe $1 million and you $250,000, which represents a huge increase in both your net worths. The gap between you has expanded. Joe is now much more wealthy compared to you than before. Question: does that mean you got poorer when I gave you the $250,000?