I don't consider English people foreigners either in the way Taiwanese consider me a foreigner. Or most Europeans for that matter. Doesn't mean I consider myself English or European. Your post makes no sense. Try again.
People identify with the place they grew up in. That's a universal. Taiwan is distinct from China. This places has had a completely distinct history for over 100 years. That matters. It matters everywhere. People have to be coerced or inculcated with a sense of national belonging to a larger unit.
Do you think Chinese Malaysians think they are part of China?
Or Vancouver-born Taiwanese, or Chinese?
Why then Taiwanese who have had a distinct history for over 100 years (and longer when you consider the immigrant experience forced very different patterns of worship and lifestyle to emerge) live under different institutions, cultural expectations, within a different cultural sphere, have entirely different shared memories going back 4 generations, and so on? You think all that is irrelevant when it is completely relevant everywhere else in the world? Why? Because of blood?
And voting patterns are very easily explained by the presence of China next door and its constant threats of war should people go too far with a push for independence.