There's illiteracy, and there's functional illiteracy. They're not the same, and people often confuse the two. A literally illiterate person (ha!) wouldn't make headway with almost any realistic computer interface, icons or not.

The 20% statistic is about people who have great trouble reading and comprehending simple sentences, not discerning individual words. It's tragic and debilitating, but such people could muddle through a simple interface with textual labels. A truly illiterate person couldn't.