That's just for adults, for children the data ranks the States against one another rather than giving an average illiteracy percentage like with adults. Here's a top tier link on the topic: https://www.thinkimpact.com/literacy-statistics
Original source: https://www.oecd.org/en/publications/do-adults-have-the-skills-they-need-to-thrive-in-a-changing-world_b263dc5d-en/full-report.html
Another review of the source material for good measure: https://www.crossrivertherapy.com/research/literacy-statistics