The article doesn't mention US citizens specifically, just US adults. But yes, the context and link make it clear it wasn't talking about South America, but even if it were, there's a table in the article that has a list of countries literacy rates. A cursory glance shows Mexico, Brazil, Colombia, Ecuador as having higher literacy rates than the US.
I don't know how accurate these stats are, but it's interesting nonetheless.
21%, yes, but what kind of American? Central American, South American?