Self-testimony in scripture cannot simultaneously be both God’s and its prophet’s words

Evidence

You (he is speaking to an imaginary Muslim interlocutor) you have to believe everything which (the Qur’an) contains to be very true. Say, therefore, when you read in siirah vi, I indeed, to whom God has shown a right way and a straight one, that namely of Abraham, who was not an unbeliever; and directed me; what do you understand by ‘I’? If you say ‘Muhammad’, anyway those words are not God’s, and they need not be believed. Muhammad proves nothing, therefore, when he bears witness to himself; and, as those words are written in the Qur’an, you have premised that they are God’s; and the two things cannot be at the same time. — Norman Daniel, Islam and the West, 1546

Categorization

• QUESTION: This is a neat logical trap, but does it actually hold against the Islamic doctrine that the Qur’an is God’s eternal speech transmitted through Muhammad? Worth chasing how Muslim polemicists responded.