There are many unique aspects and special features to Taiwan's history and relationship with Mainland China and the following comments are not judgments regarding the equity or merits of that relationship; only the propriety of the US inserting the issue as its trade negotiations ploy.

Essentially, an implied understanding now exists between the Taiwanese and Mainland Governments concerning their relationship. That understanding is that the formalities of an overarching Chinese national unity will be respected but the day-to-day independence of Taiwan respected.  Both Governments, however, implicitly retain the right within limits to try to push that relationship to its advantage.

Given how fraught and fragile those understandings are, it is the height of irresponsibility for the US to threaten interference simply as a trade ploy.  It's as though the US were to threaten to raise questions about Canadian unity as a ploy in renegotiating NAFTA.