China has a trade surplus with the US, but is net losing dollars because it has to use dollars to buy oil and other imported goods as very little international trade is transacted in yuan. They aren't choosing to shift away from the US dollar. They need them and are going through their stockpile.
[1]https://www.ft.com/content/ca50aa10-8b18-11e9-a1c1-51bf8f989... [2]https://www.wolframalpha.com/input/?i=value+of+15.6+tonnes+o...