Tibet was a self governing entity until Chinese invasion. Though China would disagree. Tibet's leaders are still in exile and one of the key issues of China with India.
If the argument is that Tibet was not a country, then the same applies to Taiwan. Taiwan is not internationally recognized as a country, except for a few nations.
Autonomy is not sovereignty. Tibet wasn’t “invaded” like a foreign country, it had been de facto autonomous after the chaotic Qing collapse, but no one recognized it as sovereign. If I were to guess at China's narrative, the PLA’s 1950 entry is probably seen as a reconsolidation of territory long claimed by China, not new imperial conquest. And Taiwan’s status only survived because US intervention froze the Chinese civil war’s outcome, not because it was ever outside China’s historical frame. Again, indeed, Qing imperialist actions 300 years ago led to the current map, and you might see me as pedantic here but calling China (or modern US/Japan/Britain for that matter) imperialist might feel satisfying, but analytically it dampens the real and harmful empire-building sense of the term used in history.
I think one issue is when do we start drawing the line that an autonomous entity is recognized as a sovereign country. Do we start with the UN? Because before nation states formed, it was a bit ambiguous. British empire's colonies were not a formally recognized countries in the modern sense. But we do agree that British were imperialistic.