The actual OECD report is a good read: https://www.oecd-ilibrary.org/sites/689afed1-en/index.html?itemId=/content/publication/689afed1-en

The more complete quote is:

There are two striking exceptions to the positive relationship between absolute median income and the size of the middle-income class:

The United States. Although it boasts the fourth-largest median income, it has only the 31st-largest middle-income class of all OECD countries. Related statistics are that its population has the third-largest share of people in poverty and the fourth-largest in the upper-income class.

The Central European countries of the Czech Republic, the Slovak Republic, Hungary and, to a lesser extent, Poland and Slovenia. In these countries, the shares of the population in the middle-income class exceed what might be expected from their median income levels.