Christian missionaries to Muslims generally don’t dispute that Mo existed. They view him as a genuine, if crazy person. It seems to make more sense than trying to explain the Koran as some sort of composite book.
That’s tactical. But while the Enlightenment crowd disputed the existence of Jesus, there is far more direct and circumstantial evidence of that outside the New Testament than the existence of Mohammed. Apart from that, the history of the rise of Islam may be wrong. The Arabs may have been allies of the Persians who took over the Persian Empire after their combined forces overran the Byzantine Empire. In other words, it was the last phase of the war between the Persians and the Byzantines which left the great cities of that Greek Empire in ruins.