Its not entirely out of possibility that China will make covert attempt.
Trump has proven to be a weak leader by not responding to China's economic strikes. The image he is projecting is someone who cannot even keep Canada or tiny Denmark in line, this is so unprecedented given these are de facto US territories. It would be like if a Chinese leader could not keep control of Hong Kong or Macau.
There is a hypothesis that the US military is much weaker than it's on paper numbers, due to old hardware and poor maintenence. Testing the hypothesis by delivering a limited version of a PLA salvo through "Yemeni" volunteers would be a low risk way of testing the theory.