they are when that one country represents nearly their entire customer base.
Sure, the US is important to China, but "nearly their entire customer base"? Ahh... EU is larger than the US to China (see http://www.stanlib.com/EconomicFocus/Documents/Global/ChinaexportstoEUvsUS.pdf). But agreed - letting US go bankrupt would definately be a hit to the Chinese economy.
Quark! Quark! Beware the quantum duck!