In what sense is China not "sovereign" over Hong Kong? While it's true that the Sino-British Joint Declaration placed some temporary restrictions on China's po

In what sense is China not "sovereign" over Hong Kong? While it's true that the Sino-British Joint Declaration placed some temporary restrictions on China's policies regarding that region, this is an agreement between sovereign nations, thus reaffirming China's sovereignty. I suppose that, in a sense, all international treaties are a loss of some degree of sovereignty, but in that sense nearly every country in the world has lost sovereignty in that time.