What does Mrs. Flowers mean when she tells Marguerite that some people, though lacking formal schooling are "more educated and even not intelligent than college professors"?

This quote comes from Marguerite's first lesson in learning. This is the point at which she learns to understand that illiteracy doesn't translate as ignorance, and that a lack of a formal education doesn't mean there is a lack of intelligence. People who are not educated have simply lacked opportunity.