I think it's a trivial observation. For example, it's clear illiterate people in Europe still knew what a cross meant when on a building, there is no doubt about that. There are many more religious symbols that were also well known. Flags and official seals similarly had well known meanings in their own areas, as did various military symbols.
OK, but symbol literacy, at least in this sense, is a long way from literacy in a language having the grammar of the spoken language of these people (or a grammar equally as expressive, as we see, for example, in the case of scholars who read and wrote in Latin while conversing in the vernacular of their time and place.)