In the beginning, the testing criteria were quite limited- history of travel to Hubei, admission to the hospital with symptoms, etc. So early on, yes, I do think HK missed a number of cases (those with mild symptoms). I would be interested in the results of widespread antibody testing, not just in HK but worldwide. It would have to be both inexpensive and accurate. I think it's the only way we will ever have a more accurate death rate because we still don't know who has had it...
Now the testing is much more widespread but early on it was probably overly-limited, I don't know if that anything to do with available test kits and labs who could process the results. I think the "spit-test" kit that is widely used now did help make testing more widespread as you can do the test at home and someone can bring it to a lab for you (if you think you may have it).