The German economy is strong compared to many other nations. Currently the German economy is the strongest in Europe.
The German economy was also for many decades before the Euro the strongest of Europe, that was one reason for the strong currency (German Marks).
The sayings of the "weakness" of the German economy was and is a political claim that was made to reduce the workers costs in Germany.
You must also distinguish between productivity (which always was high in Germany in the last decades) and attractiveness to the financial sector (stock etc.). And the second thing is, I guess, what Krugman/Muenchau really meant. And yes, Germany was not so attractive for some time, because growth and ROI where not so high as in the US for example.
But I think, that are two different things. One is about productivity and strength of industries and the other is about the capitalistic demand for more and more growth.