What the example of Hong Kong communicates is that authoritarian, illiberal, undemocratic regimes need not feel threatened by semi-independent city states with working "liberal" market institutions. It says to rulers that their countries can get rich without granting their subjects real freedom.
Which leads me to wonder: what is this "real freedom" of which you speak?
Consider the following definition of freedom: the absence of monopoly.
The absence of monopoly means that you can exercise exit, even if you cannot exercise voice. The presence of monopoly means that, at most, you can exercise voice.
Neither my local supermarket nor any of its suppliers has a way for me to exercise voice. They don't hold elections. They don't have town-hall meetings where they explain their plans for what will be in the store. By democratic standards, I am powerless in the supermarket.
And yet, I feel much freer in the supermarket than I do with respect to my county, state, or federal government. For each item in the supermarket, I can choose whether to put it into my cart and pay for it or leave it on the shelf. I can walk out of the supermarket at any time and go to a competing grocery.
The exercise of voice, including the right to vote, is not the ultimate expression of freedom. Rather, it is the last refuge of those who suffer under a monopoly. If we take it as given that the political jurisdiction where I reside is a monopoly, then perhaps I will have more influence over that monopoly if I have a right to vote and a right to organize opposition than if I do not. However, as my forthcoming Unchecked and Unbalanced argues, the reality is that the amount of influence I have is shrinking while the scope of the monopolist is growing.
The idea of charter cities (or seasteading) will be a success to the extent that it creates a viable exit option vis-a-vis government. Suppose that the Chinese government loses its monopoly power, because it becomes easy for people residing in China to choose to live under alternative governments. In that hypothetical case, I would argue that those residents are free, even if those who choose the Chinese government are not allowed to vote in contested elections or to freely criticize their government. If you lived in North Korea, which would you rather have--the right to vote or the right to leave?
In fact, if we had real competitive government, then we would be no more interested in elections and speaking out to government officials than we are in holding elections and town-hall meetings at the supermarket. I repeat: real freedom is the absence of monopoly.