I do agree that to China, Taiwan isn't about TSMC. TSMC is the reason why the US cares about Taiwan but not the reason why China cares about it. If it was the main reasoning why doesn't China go after the Dutch? The dutch have ASML, which manufactures the more critical components involved with EUV lithography. But China never made a single move against ASML.
There is deep history between Taiwan and China, and it is this history that is the main reason why China eyes Taiwan. Think of it like if California rebelled and became a separate country from the US 50 years ago. How would the rest of the US think about California?