I know it’s not their style, at least officially but it’s not like China has never done something on that front. Plus they don’t necessarily need to arm the groups, ‘causing problems’ have many dimensions.This just isn't China's style. China has a steady policy of noninterference in a sovereign country's internal affair. They see things like arming insurgent groups as the kind of thing American and Western powers do and have an aversion to doing the same thing. If China really wanted to put pressure on India, they have far more direct and pointed methods
Despite what India likes to claim, the various group are basically put down with force, supporting their goals and bringing their plight to the international stage and hammering on it would already cause a headache for India. Further it doesn’t need to be done officially, just having a means to bypass the Indians usual tactics of shutting down the internet is already good enough.