It's ridiculous these kinds of claims even make it to HN comments.
Proficient on the survey that derives from, is the highest possible ranking. Proficient on that survey does not equate to functional; anything at or above basic is functional.
About half the 14% of people that fall below functional, do so because of language barrier problems related to not speaking English as a first language.
The UK is at 21.8% for comparison.
The link you provided shows between 43% and 55% are functionally illiterate (basic or below) depending on which test you look at, for English.
Just because education is poor for people who don't know English as a first language, and just because USA services are not available in Spanish despite such a large percentage speaking Spanish does not mean there is no problem.
Functional illiteracy in the USA is a big problem. The education system in the USA is a massive failure.
[0] Cleckler, Bob (2009). Let's End Our Literacy Crisis, Revised Edition. Salt Lake City, Utah: American University & colleges Press. pp. 15–26. ISBN 1-58982-230-7.