The assumption is that they will initiate a real war against Taiwan at some point. The result of that assumption is to prepare by slowly phasing out trade where possible rather than waiting for it all to happen at once when they do invade.
And the result of that preparation is that both sides are less entangled with one another and thus (theoretically) more ready to be at war with one another.
Anyway, the chain of who-started-it doesn't have a first link. There is no value-free way to assess it all..
It's true, the US sold Taiwan down the river to drive a wedge between USSR and PRC, then spent decades telling ourselves that PRC would become a nice, (post-Christian) liberal democracy if they just became rich enough. That didn't happen.
