Thats an incredibly simplified take on it. HK might not represent a large part of the chinese economy but it still facilitates a large part of foreign investment into chinese companies.

In CN there are largely three types of listings A shares, B shares and H shares. A & B are listed on SZ and SH exchange and are not freely traded like you know it from the west. H shares are listed on HK exchange and are freely traded using the Hong Kong Dollar.

This is why companies like Tencent are dual listed on Shanghai and Hong Kong exchange, to facilitate foreign investment that wouldnt be possible in the same way if Tencent was only listed as B class on Shanghai exchange.