You misunderstand the US One China Policy. From the link you posted:

"...the United States recognized the Government of the People’s Republic of China as the sole legal government of China, acknowledging the Chinese position that there is but one China and Taiwan is part of China."

The US _acknowledges_ the Chinese position, but it does not _recognize_ it. So the US official policy is explicitly _not_ "There's only One China and Taiwan belongs to China".