The mainland Chinese government disagrees.
I don't know about these particular tariffs, but I believe the U.S. would not consider tariffs aimed at (mainland) China to apply to Taiwan.
That's a minority/extremity opinion which isn't remotely prevalent even among those who feel intimate with China. It used to be the 'political-correct' belief held by the former authoritarian ruling party. But it's not considered a rational worldview anymore after its democratic modernization.
There are things that are de facto true, even if they're not de jure true.
The status of Taiwan is one. Israel's nuclear status is another famous example.
While the common sense reaction is "everybody knows it, why pretend?" those fictions generally exist because they help maintain delicate balances and order between parties that are generally at odds with one another.
What it comes down to is that it'd be lovely if we could all just say openly and outright that mainland China and Taiwan are separate entities, because, in practice, they are. But it's also pretty swimming for the global community for the PPRC and ROC to both claim that they're the real China while everyone does business with each other anyway. So while it's true that they're distinct and it's also true that everyone knows that to be the case, everyone also wins out to varying degrees by officially pretending that it isn't the case.