Before that happens, the capital exodus demanding to liquidate their Yuan for USD would have burned through the country's USD reserves and they would be insolvent. This is quite similar to what happened in South Korea in 1997 and it took IMF intervention to stabilize the country. The PBOC could limit how much a person can sell, but this would crash their currency (e.g. Venezuela) almost instantly.
As much as people want to hate on Trump and/or the US, China is in a weaker position here. Don't get me wrong, both countries suffer, but only one of them would see its financial system collapse.