Purchasing parity power will tell you a lot about the aggregate price of tea in China but nothing about their ability to purchase Russian made weapons with Renminbis.
China's currency is artificially propped up and tied directly to the dollar. Wait until China devalues the Renminbi to protect their trade surplus. There is continual talk of that.
GDP is a more accurate statistic. Take the word of the Chinese themselves. China's National Bureau of Statistics themselves predicts that China will grow to be the number three economy in the world in 2020 and the number two economy in the world in 2050. They could not grow to number two in 50 years if they already considered themselves to be number two. The fact is they are not. Their economy is about one tenth the size of the US's as evaluated correctly by themselves.