A more robust measurement might be the (former) US Department of Education's "Adult Literacy in the United States" survey, most recently conducted in 2019. The results of this are sobering enough:

<https://nces.ed.gov/pubs2019/2019179/index.asp>.

There's a related study of adult technical literacy conducted in 33 OECD nations:

<https://www.oecd-ilibrary.org/education/skills-matter_978926...>.

Both show that only a small fraction (5--10%) of adults operate at high levels of literacy (whether of text, numeracy, or technology), and that a large fraction (roughly 50%) operate at a minimal or below-minimal rate.