No, but they may as well be. That’s why we have a third classification, “Functionally illiterate”. This is for those that technically can read, but with serious limitations. My last job had a functionally illiterate supervisor; smart guy, but could only read/write Spanish, despite being able to speak English passably. Many dyslexics also fall into this category because their reading speed is often low; they may understand what they see eventually, but not in a timely enough manner to be useful.