The author in question has acknowledged this in this book and elsewhere. In the third chapter of the third part of this book, concerning the method for determining whether a verse is clear or ambiguous, he says:

“Know that this is a matter of great importance, for every adherent of a school claims that the verses supporting his school are clear, while those supporting his opponent’s school are ambiguous. Thus, the Mu‘tazilī says that the words of the Most High, ‘So whoever wills—let him believe; and whoever wills—let him disbelieve’ [al-Kahf 29], are clear, whereas His words, ‘And you do not will except that Allah wills’ [al-Insān 30], are ambiguous. The Sunnī reverses this judgment. Examples are numerous, so there must be a fundamental rule to which one refers in this matter.”

We shall discuss the rule he mentions, Allah Most High willing. Now, if reasonable people from every group acknowledge that interpretation, as a category, must be subject to rejection, and that some of it is false and forbidden, then it is clear that this is not comparable to his claim that the groups agree in acknowledging that there must be