The percentage of the population that’s illiterate is way higher than it has any business being.

Like “can’t read at an elementary school level”.

“There are three kinds of lies: Lies, Damned Lies, and Statistics” - Mark Twain

Not in your link, but I found the same statistics as your link in another with a critical piece of information:

“According to researchers, 4 out of 5 Americans 18 and over possess medium to high proficiency in English reading and writing.” source

These statistics, both your link and mine, may only be measuring literacy in English.

So looking to get a clue how many may have literacy in another language:

“Today, 13.8 percent of the nation’s residents are foreign-born” source

So at least a percentage of those being counted as USA illiterate may indeed be literate in another language that isn’t English.