"China bids up the price of the dollar relative to the yuan by buying dollars with yuan, and then sits on these dollars by purchasing U.S. treasuries with them"

"China bids up the price of the dollar relative to the yuan by buying dollars with yuan, and then sits on these dollars by purchasing U.S. treasuries with them" Is China printing yuan to buy dollars? I thought the USD was accumulated from payments (in dollars) from exports to the USA.