Exactly SinMen, so Taiwan was never part of the People's Republic of China, so how can they say that Taiwan is a part of their state? Not like Taiwan EVER complied to the People's Republic. The Republic of China (Taiwan) even, has always had, their own republican government...so basically no, the Republic of China never belonged to the People's Republic of China.
Now if you want to talk about history, then we might as well include the various colonial forces that had controlled Taiwan at one point or another, and if we get into that argument, then you can also say that, yeah, Taiwan can also be claimed by the French, or the Japanese - and that would be a very far fetched argument.