Don't keep us in suspense... what's the theory?

I think you have a point. I am a reasonably well informed layman, but not on Semitic languages in particular, and I interpreted the Semitic bits as accepted in the field. This is a common problem with this sort of book, with the author not clearly distinguishing between exposition of accepted wisdom and arguing his case. Steven Pinker does the same thing. It is annoying. But I enjoyed the book and learned stuff I didn't know before.

what's the theory?

Too complicated to summarize here. You'll have to read the book. (He takes it in stages, and there isn't a single overarching explanation for the whole system.)