Yes, they aren’t drawing from the Bible for their conclusion. Human reasoning.
One of my responses to them is citing the verse from the Quran, where Muhammad affirms the Bible. So what the Bible says, goes.
Dr. David Wood calls that the “Islamic Dilemma”.
The Koran affirms the authority of the Bible as it was back then and as we have it now—and yet the Koran is antithetical to the Bible on just about every point of doctrine, so the Mahometan scholars and clerics have to claim that the very Bible that the Koran affirms has been “corrupted” without any evidence to back that up.
Therefore, if the Bible is true, then obviously the Koran is false—but if the Bible is false, then the Koran is also false.