One of the points in it is that Cortez easily convinced the slave tribes to support him against the cannibal Aztecs.
The Aztecs fought smart, hard, and courageously. Just because they were evil does not mean they were stupid or cowards. They were extremely effective warriors, who came up against superior weapons and technology, and a superior religion.
We should not discount the part that disease played, either.
One thing I learned about Spanish conquest of the Southwest is that simply by walking through a settlement, the Spanish could destroy it. They would return a year later and find it ravaged by disease. They more or less accidently depopulated the entire region before they even attempted to colonize it.