A rose by the same name smells just as sweet. (Sorry Shakespeare!) But honestly if the Arabic word "Allah" is used by Arabic speaking Christians for the Christian God, how in the hell can they be thinking the same Allah as Muslims?
I'm going to regret this, aren't I?
They're not in anyone but Theophilos' mind. I remember a while back I had a conversation with a Coptic friend who was railing against Arabic, and how much he hated it and he hated his forefathers for giving up and learning it, so that he now has to speak it as a native language. I pointed to the Agpeya/Horologion on his desk, which was in Arabic, and he said he prayed from it every day. Then I asked him if he read the Bible in Arabic, and he said of course. Then I asked him how he could hate Arabic when it was through Arabic that he prayed to God, and that he studied and embraced and loved God's word? He thought about it for a second and apologized for not thinking it through before becoming upset.
Other than giving appropriate responses during the liturgy, I do not pray in Arabic despite attending an Arabic speaking church (which does ~75 to 80% of its liturgy in English...), but it would be ludicrous for me or anyone to cast aspersions upon the Christianity of those who do. That kind of bigoted, xenophobic, borderline racist nonsense has nothing in common with the true Orthodox faith of God, in which there is neither Jew nor Greek (and, I would say, neither Arab nor Ajam).