Those are literacy rates, which are distinct from literacy proficiency levels.

From your link:

Literacy rates display the % of adults ages 15 and above who can both read and write with understanding a short simple statement about their everyday life.

That definition corresponds roughly to the Programme for the International Assessment of Adult Competencies' definition of being below the lowest level:

At Level 1, they can understand short texts and organised lists when information is clearly indicated, find specific information and identify relevant links. Those below Level 1 can at most understand short, simple sentences.
– OECD Survey of Adults Skills 2023: United States

Overall, people overestimate average literacy levels. The US is slightly worse than the OECD average of 26% of adults at Level 1 or below. Even in the highest performing country, Finland, 12% of adults meet that definition.