In a free market, no one truly knows how the market will respond to some signal (like the purchase of bonds). Although an unrestrained manipulator (The Fed) can get pretty close to setting the price of all bonds by buying huge quantities of certain bonds -- The Fed doesn't know with certainty how that will affect the price of the currency (USD). And they don't really care that much, to be honest. They don't measure inflation with respect to a basket of currencies. They measure inflation with respect only to the USD.
When China literally sets the price of the RMB, there's no question what will happen to the price of the RMB. It is whatever China says it is.
There's a huge difference between manipulating the price of your currency and your bonds. And it's because there's a huge difference between what bonds and currenies are, and how they're used.
