Your assessment is probably closer to the truth. 54% of American adults have a literacy below sixth grade level link and some of the people you’ve met probably are considered barely literate yet counts towards the 79%.
A curious statistic I’ve found while reading up on this is that 77% of African Americans have moderate or high reading proficiency while only 65% of white Americans qualify as such. A statistic that you’ll never see racists mention (and libs for those that somehow fit outside the venn diagram)
It was just a cute factoid that I noted, so I didn’t look further into the claims.
Your theory could be correct. Another reason I suspect is that due to racial biases and different job market situation arising from the urban/rural divide, black Americans are forced to be more literate in order to survive compared to the average white American.