I only looked at the abstract, and only for like 3 minutes, but it looks like this is the relevant line:

Four in five U.S. adults (79 percent) have English literacy skills sufficient to complete tasks that require comparing and contrasting information, paraphrasing, or making low-level inferences—literacy skills at level 2 or above in PIAAC (OECD 2013). In contrast, one in five U.S. adults (21 percent) has difficulty completing these tasks (figure 1).

So not that 1 in 5 adults can’t read, but 1 in 5 adults have difficulty completing basic tasks involving reading. OP’s choice of the term “illiterate” seems like a poor choice, but that also seems like a never ending pedantic argument that I’m not really looking to get invovled in.