Medieval Christians saw the Qur’an’s unlikeness to Scripture as its real disproof.

Evidence

Although this Muslim accusation was widely known, it was also said that the Qur’dn confirmed the Scripture, which, quite unjustifiably, was taken to mean the existing Christian canonical Scripture. Arguments to defend Scripture were illogically based upon Scripture, but the real disproof of the Qur’an was seen to be its unlikeness to, or contradiction of, both Scripture and philosophy, and also in its being contrary to reason. — Norman Daniel, Islam and the West, 1515