If you count Taiwan as a part to be unified under CCP rule, then by similar accounts you may as well argue that Iran and Iraq as parts to be unified under CCP rule.
From the Wikipedia page on Taiwan [1], check out the Chronological phases on the right, you can see that Taiwan has been ruled by different regimes:
* Prehistory to 1624
* Dutch Formosa 1624–1662
* Spanish Formosa 1626–1642
* Kingdom of Tungning 1661–1683
* Qing rule 1683–1895
* Japanese rule 1895–1945
* Republic of China rule 1945–present
Among them:
- the Kingdom of Tungning was a loyalist to Ming dynasty when Qing dynasty was ruling mainland China,
- later Qing dynasty partly ruled Taiwan,
- now the Repulic of China is ruling Taiwan when CCP is ruling mainland China.
Then one may ask why Taiwan should be a part to be unified under CCP rule, as Xi implies, instead of being ruled by the Republic of China.
One argument is that Taiwan has been ruled by regimes associated with Mainland China in the past (loyalist to Ming dynasty after it lost, then partly by Qing dynasty), hence by extension to be ruled by the current ruler of mainland China, CCP.
But if you accept this argument, then you may notice that the Mongol empire (Yuan dynasty) once ruled Iran and Iraq and more [2]. If we ignore the past, then we may ask if Taiwan should be ruled by the mainland Chinese regime, CCP, or should be a sovereign country ruled by the Republic of China as the President of Taiwan said [3].
[1]: https://en.wikipedia.org/wiki/History_of_Taiwan
[2]: https://en.wikipedia.org/wiki/Mongol_Empire
[3]: https://timesofindia.indiatimes.com/world/rest-of-world/taiw...