How is that a fact? Since only traditional Chinese is taught in HK and most signs and publications are in TC. Like Mainlanders and Taiwanese, HK people would have to learn the other character on their own.
The difference between SC and TC is not that great, as many SC are simplified using same rules. One can also guess the odd ones using context of the sentence.
How do you extrapolate one instance of one mainlander who didn't know how to fill out Custom Form to make conclusion a whole population? What about the other Mainlanders on the flight who knew how to fill out the form? Sometimes new travelers who are native English speakers have trouble filling out Custom Form too.
Let's just say that I know enough Mainlanders and Taiwanese to come to that conclusion.
The Custom Form is issued by CBSA (in English and French).