So I am a researcher by trade in this field, got a PhD, and develop these kinds of stats (at a more local level). I also have taught basic adult literacy for about 15 years. I think the poster was likely referring to an NCES stat.

We tend to think of adults with low English literacy as people who dropped out of school or never went. We also tend to think “illiterate” is binary, you can read or you can’t. But the definition is based around grade-level reading (what can you identify and synthesize from standardized text in English in a given time frame) and inclusive of a broader population. We’re talking about people who can’t pick up a copy of USA today and tell you the main idea of a front-page article. They can drive, they can work, etc. So they get along and this issue get ignored.

For example, some stats on illiteracy will count “non-participants” among those who can’t read/write, but this includes people in the study with cognitive disabilities or language barriers to the point that they can’t take the reading test. The share of U.S. adults who are functionally illiterate in English includes some non-native English speaking adults and also a couple generations of folks with reading diasbilities who passed through school, AND people who didn’t read for myriad other reasons.

I have tutored older adults learning to read/write for many years and have met a lot of people who ran businesses or raised families or worked full careers before learning to read. Adaptable and clever bunch. And even many U.S.-born native English speakers who got shuffled through high school despite serious disadvantage and/or disabilities.