It's a very bad tool for that particular job, as we discovered in Iraq -- it hurts the most vulnerable members of the society, and does very little to the elites making the policies we don't like.
In this case, sanctioning Hong Kong is part of sanctioning China, because it hinders technology transfer (eg to Huawei) and finance transfer (Hong Kong was previously not affected by trade tariffs on China, and has a huge stock market).
It remains to be seen if sanctioning China (partially) could bring about political change, as CCP is endorsed within China mostly for economic growth in the past three decades.
After decades of sanctioning Libya the united states finally got what it wanted by decapitating the state with violence; one wonders (global political dynamics aside) if for the unfortunate citizens of Libya, it would have just have been better to have gotten it over with earlier.
