The first question was fine of course but the political flamebait in the rest predictably took over.
It is. And it's nothing new and well-known[1]. OP is Fermats library's high resolution picture of the 17th century reproduction.
District 13
Says Americans, biggest lovers on this planet for their own national flag. /s
This is the claim that China and Taiwan should reunite since the PRC drove the Nationalist Party out of mainland China. It isn't fictitious, it's merely a political wish to merge the two from the PRC side. Similarly, some Nationalists in Taiwan are happy to take back mainland China, as much as the country is divided on whether Taiwan should remain as it is, or should declare its full independence.
I'm very skeptical of purely historical claims of sovereignty. If the people of Taiwan legitimately vote to join the PRC, that's their right, but the PRC has no legitimate right to demand it.
[1] https://en.wikipedia.org/wiki/Political_status_of_Taiwan
[2] https://en.wikipedia.org/wiki/Political_status_of_Taiwan#Arg...
[1] https://commons.wikimedia.org/wiki/File:ROC_Administrative_a...
https://en.wikipedia.org/wiki/Confederados https://www.vice.com/en_us/article/welcome-to-americana-braz...
There were also settlements in Belize and Mexico apparently. None of them lay claim to being independent nations and owning mainland US though.
The ROC as I see it retreated into an enclave of the former whole China and was never fully vanquished from Chinese land. So it's really more like if the Union stopped fighting after the Confederacy was defeated in all but one breakaway state. Even more accurately, since the PRC were the rebels, it'd be like the Confederates took over all the Union states except, say, Pennsylvania and then the war came to a halt. The Pennsylvania redoubt of the US would still be the US and may or may not make claims to the rest of the formerly united United States.
But what do I know about the situation? I'm just an American.
So it's not as if the ROC is free to amend their own constitution - as doing so would negatively affect their national security. It's safer for them to keep up the ridiculous notion that they have sovereignty over Mongolia and parts of the Russian far east.
China really doesn't want to find out what might happen if countries start recognizing Taiwanese independence, so the US president poking at that bubble is seen as a threat. China responds with assertive words because fighting with words is better than fighting a war.