Or they may just be pushing back on the idea that the PRC has legitimate claim to the nation of Taiwan. People online like to say it because they know it upsets the PRC government. Basically, it asks the question, “What makes the PRC any more the “True China” than Taiwan?”
Truthfully, neither nation is “true China”, and neither are the nations that they were years ago. No one in Taiwan today holds any belief that the ROC government is the rightful government of the mainland in exile.
But Taiwan is unable to be widely recognized as a sovereign nation in its own right to this day because the government of the PRC is still sticking to the “manifest destiny” sort of idea that there is a single, ideal land of China rooted in its imperial legacy, which for some reason the current mainland government feels they have an obligation to claim.
In one sense, as a key component of the UK, they already had that chance somewhere between the years of 1600-ish to 1945-ish.