If it's Quranic Arabic then the orthography includes vowel markings so vocalising it is considerably easier.

The number of people who can vocalise Arabic without the vowel marks (e.g. a newspaper) is considerably less. But you don't need to be able to do that to notice any of the errors in the article.

A quick Google search seems to indicate that there are 1.9 billion Muslims in the world. It makes sense to assume they know enough Arabic to recgonise at the very least common religious set-phrases like the Bismillah or the Shahada. In my book this counts as "some degree of Arabic script literacy"

You'd have to exclude everybody under 7 or so, everybody who is illiterate (in this very expansive sense), and those that are Muslim but do not read Arabic and do everything in translation.

That might be a small group though and probably outweighed by all the non-Muslim Arabic readers (for instance I work with 2 Egyptians, one is Coptic and one is ex-Muslim and both can easily read Arabic).