Look, first off, if they are traveling with a Diplomatic passport, they will not travel without having one already in their possession extending at least 6 months beyond the expected duration of their assignment. On top of the passport, they need to get a special diplomatic visa. The concept of diplomatic immunity is basically a gentlemen's agreement. There is no RAMBO force field shielding diplomats. The Chinese are shifty fucks and like to send messages and the best the US or any other country can do publicly is send out those strongly worded letters.