Is this another attempt at irony? Or are you saying that the 7th century Muslims were so illiterate that they could not tell the difference between the names Issac and Ishmail?
Genesis 16.11-12 doesn't have this problem. It says an angel of the lord came to Hagar and said:
“You are now pregnant and you will give birth to a son. You shall name him Ishmael, for the Lord has heard of your misery.
12 He will be a wild donkey of a man;"
Maybe the Arabs do not want to be known as descendants of a wild donkey of a man and pretend Ishmail is Issac.