First of all, I am not aware of any comparison that Diamond makes between China and the United States in terms of unity and disunity in this book. Diamond does emphasize the impact of China’s unity, but he contrasts it with the disunity of Europe, not of the United States. As Diamond says on page 414 of the paperback edition, in order to understand China’s loss of
…technological preeminence to Europe (you must) …understand China’s chronic unity and Europe’s chronic disunity.
The argument that Diamond makes is that China had no real competitors in its region and therefore did not have to make progress technologically. By contrast, European countries had many competitors and therefore had to make progress.
So what would Diamond say about the US? Like China, the US has had no real competitors in its own region for a very long time. However, the US started to grow during an era of global competition. It actually did have competitors even if it was not competing with countries in its own region. The US had to compete with England, Germany, Spain, and eventually Japan, among other countries. Because of this, the US had to become more technologically creative in order to keep up.
In other words, the US always had competitors, even if they were not near neighbors, while China did not.