A notable effect that skews those numbers a bit is that the test for literacy is often given in English only, meaning any people in the US who can read and write Spanish but not English are counted as illiterate. (It's slightly more complex than this, some states let you take it in Spanish, some have the option but still usually give it in English most of the time, but the effect is the same)
That's a really good catch. It looks like that 28% figure is specifically Level 1 or below in English literacy, which makes that number a whole lot less interesting for evaluating education levels.
I care about ability to comprehend text in someone's fluent language. The PIAAC figures don't appear to measure that.
Do you care about trends? Like an increase from 19% to 28% between 2017 and 2023?
Sure, but if that trend turns out to be an increase in non-English speakers it tells us less about the state of literacy, which is the thing I'm interested in here.
The national results page literally has a section where it shows separate results for native-born and non-native born participants. You claim that you're interested, so it's strange to see you speculate about what "turns out" when the answer is so conveniently available.