Hong Kong's importance to China isn't its GDP contribution primarily (although it is special vs China when it comes to per capita performance; Hong Kong packs a lot of economic punch in a small package). It's the special relationship Hong Kong is still granted globally on trade & finance, particularly with the US.

China desperately requires dollars to keep its debt problems floating and to interact with the global economy (relatively little is settled in Yuan outside of China's borders). Without a steady inflow of dollars China cannot function normally; their economy would seize up.

Hong Kong is a critical inflow port for US Dollars into China's economy and is largely spared from the tariff war.

This is why Hong Kong is so important to China right now, and it's also why China has been so careful in suppressing the protests (by their standards). China is afraid of the US changing Hong Kong's special status. People often don't think China can be swayed: the US slapped currency manipulator status on them, and they immediately backed off from letting the Yuan sink, carefully pegging it just inbounds near the 7 line.