That's not all there is to it. There's the aspect that Taiwan has been independent (de facto) from China for more than 50 years, has a completely different political system, government, and has also evolved because of this a distinct cultural attitude (similar as they are on other matters culturally).
Then again, I'm guessing you think Kosovo 'belongs' to the Serbs, right? Pfft.
That analogy would only work if the mainland chinese would consider Taiwan as the birthplace of the chinese people and culture. Kosovo reminds me more about the Israel/Palestine-conflict.