The author is defending the single number UE rate as a measure of an economy's well being. His core point, some countries don't encourage working to the same degree as the US. In other words, some countries policies instead encourage people to stay home (and care chldren, for example). Thus, the labor force participation rate for such a country (Germany, in this case) is lower. So, in this sense, to know if the economy is doing well, the UE, as measured against the labor force, is a better single measure.

Germany's business cycle...blah blah blah.....(150 words!)