This is a common misconception. The reality is that local sources cause the majority of Hong Kong's air pollution. Marine, road transport, and power generation are the three primary contributors.

Marine contributes an astonishing amount of SO2 and NO2 and the worst part is that is easily solvable. While most developed countries wont allow ships to burn filty bunker fuel within ~100 miles of the coastline, HK allows them to burn it for days while sitting at port. There was an article a few years back about how the largest container ships each produce an equivalent amount of air pollution as 50 million cars when burning such fuel.

Several logistics companies voluntarily switched to cleaner fuel on a trial basis. The cost difference puts them at a disadvantage to their competitors and they have urged the government to force the requirement on all. Without it being mandated, they will be switching back shortly. Predictably, the the government has refused citing a "need to coordinate with neighboring regions". No doubt they will put a committee together to study the potential to create a preliminary discussion document due 2033.