The conclusion is based on incomplete science. Also, it's relevant to note that schizophrenia is often not diagnosed if there aren't hallucinations and a born blind person would be unlikely to have visual ones at least, even with other symptoms. Not saying that it is required for diagnosis, just that many providers would be less likely to diagnose schizophrenia rather than other similar diagnoses without that symptom. Auditory hallucinations are actually the most common, but those without visual hallucinations can be attributed to quite a few other conditions, some of which are physical rather than mental.
Anyway, the studies are very limited and incomplete and there are other things that could explain the correlation rather than it being causal.