That was in the Cold War era, but conditions are very different today. China might still be reemerging, but it's already a bonafide great power, and great powers dance around each other very carefully. Overtly hostile acts, like actively supporting separatist elements, especially ones that employ terrorism as SOP, are too provocative and risky. I have no doubt both countries are conducting covert operations against the other, but not something that can cause innocents to die, especially not actions that could easily be traced.