If 90+% of whatever society you are from was killed off by disease, and the people who came across the survivors treated them as little better than cattle and systematically destroyed any remaining trace of your society, how literate would history judge you to be?
Literate has a definition that requires the ability to read and write. The North American native population was, by definition, illiterate. They had no written word.
It doesn't mean they were stupid, they obviously picked it up quickly when they were exposed to it
See my quote from the Britannica.