I'm sorry I can't add much more detail as I'm no expert and I'm paraphrasing an off-hand comment I heard around the time the book was published. The short of it is that Bowen & Lewis seem to take grammatical positions at odds with the rest of the field. This doesn't necessarily mean they're wrong. As you are likely aware, the Sumerian language is poorly understood and there's plenty of disagreement among Sumerologists.