It's an approximation of how many Muslims there are in the world, making the generous but not entirely incorrect assumption that they will be familiar enough with Arabic to at least be able to tell if you've completely borked your rendering.
I know Muslims who would be challenged to speak more than 3 words of Arabic, as well as Arabic speakers who are Christians or atheists.
Source: my grand aunts saying "arapreme" where "ora pro nobis" would go (the former kinda sounds like an Italian word). Also probably related, "hocus pocus" is the "magic" expression "hoc est corpus" ("this is the body [of Christ]").
I don't know if it's true but that does not seem unreasonable.
> that does not seem unreasonable.
My point is that what seems reasonable to someone that doesn't know anything about the subject means nothing, and could even end up being somewhat offensive.
Try to reverse the logic and realize how absurd it would look to you: say a guy in middle east wanted to roughly estimate the number of Latin speakers and made the assumption that it should roughly be the same as Catholics.
Wikipedia says there's ~2 billion muslims, and 400 millions Arabic-dialect speakers (native and non native). So on average 20% of muslims are able to understand basic arabic, I expect that not even half of those would be able to read and understand classical Arabic such as written in Quran.
With few exceptions, Islamic revelations do not state which Quranic verses or hadith have been abrogated, and Muslim exegetes and jurists have disagreed over which and how many hadith and verses of the Quran are recognized as abrogated,[6][7] with estimates varying from less than ten to over 500.[8][9]