I can't comment on the overall accuracy of the theory as I haven't read the arguments in the book, but what I gleaned from the article doesn't convince me. Its relation to my hobby-horse, schizophrenia, while more convincing than the autism-schizophrenia dichotomy posed by others, isn't perfect, and the analogy between the primitive bicameral mind and the schizophrenic fails in a number of places, I think.
For one, the schizophrenic does seem to exercise executive control over his autobiographical narrative (at least as much as any of us do), but the problem is that he is including his hallucinations into his mental schema for understanding the world. Although one of us might create a tulpa, we don't actually include the tulpa into our worldview except, perhaps, as a sort of mystical addendum (much like someone religious would do with his religious experiences); that, or we explain it as a self-caused mental illness/delusion.
The problem in the schizophrenic, therefore, isn't the hallucinations or delusions themselves (many people have visual and auditory hallucinations without schizophrenia), but a problem in structuring his world narrative. I've posted on /so/ about how the more we believe in things, the more we tend to see of them, and something similar seems to happen in schizophrenics. His hallucinations continually verify his worldview, and he begins to fit the events in his life all into this construction, resulting in a positive feedback loop. Over time, there seems to be a certain absolute realignment in perception, skewed by his new personal narrative--and after that realignment it's almost impossible to return to a more normal view without something to massively disrupt the thought processes in the brain. Schizophrenia seems to be some combination of being predisposed toward having these hallucinations and one's openness to the explanations for these hallucinations offered by the environment and experience (resulting in the kaleidoscopic bricolage of elements you generally see in schizophrenic worldviews).
Then again, this too is mostly speculation. No one really knows.