This may be referring to people who can "read" the Koran, where "read" means say the text out loud. Understanding what they're saying is a different question, and most of those people probably do not understand the Koran's language (which is a more than thousand year old version of Arabic).

Alternatively, it may be referring to people who read a language written in Arabic script, but not necessarily the Arabic language. Languages that use some variety of Arabic script include Persian (Farsi and Dari), Urdu, Pashto, Western Punjabi, Uyghur and some other languages. Some of those languages use vowel diacritics, particularly Uyghur.