This is a very nice piece of propaganda. You’re correct insofar as the former colonial power behaved very badly indeed. But the argument that “because X was rotten Y has every right to be rotten too” is based on a very cynical view of affairs. Your idea that it is illegitimate for ordinary people in Hong Kong want to cling on to some measure of civilized autonomy in the face of a repressive regime is, frankly, indicative of the same sort of reasoning normally condensed into the saying “might makes right.”
Of course China will crush the demonstrators. Of course Hitler burned the Jews. Of course Stalin and Mao exterminated tens of millions on a whim. When you have absolute power you can do whatever you want. But none of these acts contribute in any way to civilization. China will ultimately fail for the same reason: repression is easy, but building the future requires a diametrically opposed strategy. And no amount of social media propaganda can ever change that fundamental fact. So China will win a Pyrrhic victory in Hong Kong at the cost of throwing away its own future greatness.