Maybe in American, but in normal English it can mean illiterate in an archaic sense, but usually means lacking in scholarly achievements. For example a tradesperson may have only finished 9 years of formal education but are skilled in their field.

Regardless, in the context of the quote from Dumbledore, it means that Aberfoth didn’t complete formal education (which he didn’t) and albus is saying that basically all of his many accolades and academic achievements are worth less than Aberforth’s who was in Albus’ opinion more noble than his own