Of course China could argue this, it would be really stupid, but nothing prohibits countries from making stupid arguments if they want to push their agenda. In this vein, the Wall Street Journal told readers that if the U.S. took actions against China and other countries for deliberately depressing the value of their currencies by buying dollars they:
"...could argue that Federal Reserve policies that weaken the dollar qualify as subsidies."
Obviously they could make this argument, but it makes little sense. There is a clear difference between central bank policies designed to affect the domestic economy and policies that are designed first and foremost to affect the value of the currency. It really is not hard for people to understand the distinction between a central bank buying its own country's bonds and a central bank buying the bonds and assets of other countries. This is about as sharp and clear a distinction as imaginable.
Central bank policy will affect will the value of the currency, but so will other policies. For example, a large reduction in government spending would be expected to reduce output and lower interest rates. Other countries could with equally good grounds contest this cut in government spending as an unfair subsidy.
This is a clear case where the Wall Street Journal does not like a policy and is inventing reasons for its readers to go along with its view.