Just look at the adherents who follow them and the books that were left to instruct them.....
Darkness and Light
“allah and Yahweh (Jehovah) cannot possibly be the same God as many people think “
Jesus told the Samaritan woman that they worshiped God in ignorance, and Jews worshiped the God they know. Paul said something similar about the pagans worshiping the true God ignorantly (i.e. the “unknown God”).
As Christians, we recognize that not every person who identifies as Christian is truly a follower of Christ. But we don’t insult the “Christ” of false Christian groups but rather seek to distinguish the errors from the truth about Christ.
It is better to address how the Muslim view of Allah / God is wrong, rather than accept their view of Allah / God and insult who they call “Allah”.
It is equally possible for “Christians” to worship a false Jesus, but we do not insult the name of Jesus or the titles of Lord or God just because other groups misappropriate them.
Arabs (even Muslim Arabs) can receive salvation just like any other people, by calling on the name of the Lord Jesus. They need to understand that the true Allah is not the God written of by Mohammed, but the Father of Jesus is the true God / Allah. And Allah is not His name. It is His title in Arabic though.