There isn't much practical impact in terms of the affected trade being made to switch to "Made in China" despite the tariffs. HK made goods, as in those that actually originated in this city, are tiny in volume and any exports to the US are minimal. Not a surprise since HK today is predominantly a service-based economy with little of the manufacturing it once had. The vast majority of HK trade to the US are re-exports instead.
This is more a signaling move. The US earlier said they have ceased recognizing HK as enjoying separate status from the mainland. They are now taking concrete steps to implement this new policy. The real concern isn't this move, but any later US action that can potentially inflict real harm on HK economy and financial hub status.