I think the usual argument is that Hong Kong is much more attractive as a meeting place for foreign and domestic businesses than other cities (in no small part due to the Great Firewall getting in the way of corporate communications). It might represent a small fraction of China's GDP (or whatever other measure we're using for economic significance, since IIRC attempting to calculate China's GDP is a pretty wonky affair), but if it stops being that place where domestic and foreign businesses meet (and other cities fail to take up the slack), it could make things a lot harder for operations in other cities to continue.
That is: HK's value is less about its own numbers and more about the numbers it enables other Chinese cities to achieve.