He’s talking about the case for inflation, which is generally viewed as a bad thing. It conjures up people losing their life savings, watching the value of their earnings erode, and so forth.
But economists have known for some time that modest inflation is a good thing. It helps people in debt and it stimulates the economy. Krugman draws the obvious point, once you think about it, that moderate inflation is good for the poor, bad for the rich. If the value of money declines, the value of what you owe declines – and that’s obviously better for the poor as it makes it easier for them to get out of debt.
Not so good for the rich. If the value of money declines, the rich will be less rich.
So that is what is “unsayable.”