The problem with the use of “Allah,” is that 99% of the world doesn’t know of the existence of Arab Christians. This includes the Muslims who would think that the pope is acknowledging Mohammed’s false conception of God.
That problem is not with the use of “Allah” but with 99% of the world lacking education. In the case of the Muslims in particular, it would do them good to understand that they do not have a monopoly on the faith in Allah.
I actually doubt that the Muslims “don’t know of the existence of Arab Christians” — they live with them, sometime peacefully, sometime not. In Iraq, Syria, Egypt, Arab Palestine Christians are a noticeable minority, and in Lebanon they are close to half the population. These are all Arab countries are they not?