Sorry, but if the economy of any nation with 100,000,000 residents is so affected by a $20,000,000,000 source of money, that nation is not rich.
I think you misunderstood who the article is saying is rich. It is not the country as a whole it is its elite ruling class. The abundance of natural resources in Mexico combined with an abundance of labor give it a potential for wealth that is astronomical. But the rich sit on their wealth and the government, which is essentially controlled by them, is too corrupt and bassackwards to make use of those resources.