If the Chinese had been less self-sufficient, or more interested in the wares of Europe, they had the access and technical skill to have colonized California a thousand years before the Spanish. However, this action was a tremendous drain on the royal treasury, and was not successful in preventing Chinese merchants from losing large sums of money, especially to Japanese pirates. When the Portuguese entered into the Indian Ocean for the first time, they found it virtually empty of ocean going ships. The Portuguese never appreciated that the Chinese had vacated it just a few years before. The spread of man throughout the Americas separated the human race into two effectively non-communicating groups with different technological development histories.
Technological development is largely a matter of chance.
For example, metallurgy is discovered when an unusually hot bonfire is located on top of earth containing metal ore. The genius of man is in his ability to replicate and communicate such chance discoveries. European explorers saw the California natives as lacking metallurgy, written language, mathematics, gun powder and many other technological achievements and assumed the natives were mentally inferior. The Americas were technologically behind the Europeans but they were on a roughly consistent pace of progress on a per capita basis. In a direct parallel to early Europe, which received substantial technology through trade and war with the peoples of the near east, the people of California were probably not far from receiving the same sort of technology transfer from the Aztecs, despite their physical isolation.