For example: Japan shares many characters with Chinese, and Korean and Vietnamese both used to use Chinese characters, but they are all separate languages. Similarly, numerous "dialects" in China are less mutually intelligible than many different Western languages. Compare Portuguese and Spanish to Cantonese, Mandarin, Taiwanese (Hokkien), Fuzhounese, Shanghainese, etc. Not to mention languages like Mongolian (see the Yuan dynasty). And that doesn't even begin to get into even more ethnically diverse regions like Tibet and western China.
Then there's also the "warring states period" when, for over 250 years, no fewer than 14 different independently governed states made up what is today eastern China.
Claims that China has always been one civilization are politically expedient versions of history that glaze over an enormous quantity of inconvenient facts.
Did you know that after the Ottoman Empire conquered Constantinople, Sultan Mehmed viewed himself as the successor of the Roman Empire and declared himself "Caesar of Rome"?
China today is as much a continuation of those previous states as Turkey is of the Roman Empire.