The Government may want to strike a blow against tariffs by devaluing, but it also makes a lot of people want to get their money out of China, before it is devalued further.
If they roll tanks into Hong Kong to put down demonstrators, they could have full blown capital flight, and uncontrolled currency collapse.
And if the PLA went into HK they could go into the banks and get the CCP all the foreign exchange it would ever need.
Would kill the Golden Goose, but they might think it worth it.