But the point of that war was that the Sui dynasty was attempting to re-assert its control of Korea (and even then China did hold on to much of Korea anyway), which had been customary under several dynasties for many hundreds of years.
And Korea again fell to the subsequent Tang dynasty (668 AD).
The point about holding Korea did not imply that China held it continuously or always completely.
The problem is that the Chinese seem to feel (as the Muslims do, for example about Spain) that if China EVER controlled a piece of territory or briefly explored there (Tibet, Korea, Taiwan, South China Sea) that they own it forever, regardless of the desires of the people who actually live there.