> Under the one-china-policy endorsed by the United States Taiwan is not a sovereign state
That's incorrect.
The US does not endorse that Taiwan is owned by China and that there is only one China (which includes Taiwan), in the terms that China recognizes it. The US policy is to recognize China's position, and that is all it represents. If your premise were correct, the US would not be selling military weapons to Taiwan to defense itself from China.
This explains it moderately well:
https://www.bbc.com/news/world-asia-china-38285354
> What is the 'One China' policy?
> It is the diplomatic acknowledgement of China's position that there is only one Chinese government. Under the policy, the US recognises and has formal ties with China rather than the island of Taiwan, which China sees as a breakaway province to be reunified with the mainland one day.
> The One China policy is a key cornerstone of Sino-US relations. It is also a fundamental bedrock of Chinese policy-making and diplomacy. However, it is distinct from the One China principle, whereby China insists Taiwan is an inalienable part of one China to be reunified one day.
> The US policy is not an endorsement of Beijing's position and indeed as part of the policy, Washington maintains a "robust unofficial" relationship with Taiwan, including continued arms sales to the island so that it can defend itself.