Talman and Travis, the editors of UIM, subsequently posted an irenic response. Both the original review and the response are worth your read. Here is one quote that stood out to me in Talman and Travis’ response (emphasis mine):
Waterman also raises the question of why we did not include some of what he sees as the more controversial ideas which some have associated with insider movements. He mentions, for instance, having a very high view of Mohammad, holding to a low Christology, placing citations of the Quran next to the Bible as a source of spiritual authority, and an overemphasis on the compatibility between Islam and Christianity. Frankly, it never crossed our mind to include these ideas in UIM as we do not see them as core or inherent parts of understanding insider movements. Our aim throughout the book was to emphasize principles and practices that seem integral and common to most insider movements.
I appreciate the discussion this has generated and I think we all still have a lot to learn about the insider phenomenon.