As I understand it, Germany relies on exports and foreign investment?

It doesn't work if every country does it. In order for some countries to export more than they import (resulting in savings invested abroad), other countries need to import more than they export and pay for it by satisfying international demand for investments.

So, on the one side you have counties like Japan and Germany and on the other you have the US.

The demand for savings is apparently what drives this. People need to invest their savings somewhere. If "safe" investments aren't available, financial markets will somehow invent them. This may or may not actually be safe or productive.

My statement was in response to the blanket question "how did we come to the point where not being indebted is bad?". The (somewhat blanket) response was that debt is not always bad.

Second, I second Krugman's conclusion that rebus sic stantibus, Germany should borrow more (very cheaply) and invest more (for example, in fixing the shamefully neglected infrastructure). This a fairly specific statement, tied to a place and time.

Germany's infrastructure is "shamefully neglected"? Compared to what?

(I'm in the US, so maybe my standards are too low...)

Neglected compared to the past. Many bridges and roads are in bad repair. These are all very long term assets, and with rates as they are it seems a good time to repair them now. (Unemployment is relatively low in Germany, but high elsewhere in the EU.)