Interesting turn of events. Amazon AWS recently opened a region (Apr 19) in Hong Kong. I wonder if it'll have any impact for them and that industry in general. Perhaps Hong Kong will be inside the great firewall in the near future, too.
That said, they can restrict access to externally hosted content for users in Hong Kong, and threaten seizure of assets in HK if the target doesn't comply.
There's a lot more extradition treaties with HK and the PRC than there are with NK. Any country that China can exert significant pressure on could detain and extradite you if China decides you have broken the HK NSL.
It does according to Europe and their extraterritorial law called GDPR.