It all comes down to enforcement. Without enforcement, laws and authority are meaningless.
Take the USA and England as analogous to Taiwan and China.
In the 1700s, England had a "One-England policy" and claimed authority over the US colonies. The US colonies disagreed. Some of the international community acknowledged the One-England policy. Some didn't.
At the end of the day England tried to enforce their authority/laws and lost. Thus, England actually didn't have authority over the US colonies.
I suspect something similar may happen someday with Taiwan (though hopefully with less violence than the American Revolutionary War).