My immediate reaction is to look at different cuisines to prove or refute this hypothesis. China has a huge population who grew up eating food virtually devoid of dairy. And then in Indian cuisine there is plenty of cheese (paneer), cultured milk (raita) and milk in chai and desserts. Rice is gluten-free and is the main starch staple in China. In India they eat a lot of naan which has gluten. So if that theory were true, you would expect far fewer cases of schizophrenia in China than in India.

And yet

"For China, the estimated prevalence of schizophrenia in 1990 was 3.91 per 1000 population, so there were an estimated 4.43 million prevalent cases. This prevalence is less than half that estimated for developed countries (8.98/1000), but is similar to that estimated for India (3.36/1000)." Characteristics, experience, and treatment of schizophrenia in China - PMC).

Another reaction: the author attributes a contribution to developing schizophrenia to modern anti-diabetic medications Tradjenta or Januvia. But were these medications even around when the term schizophrenia was first coined?

"The introduction of the term and concept schizophrenia earned its inventor, Swiss psychiatrist Eugen Bleuler, worldwide fame."
"Eugen Bleuler (1857-1939)"
Eugen Bleuler's schizophrenia—a modern perspective
- PMC.

Tradjenta was first FDA approved in 2011
Tradjenta (linagliptin) FDA Approval History - Drugs.com

So that casts doubt on the hypothesis. The disorder is not new and yet modern medications are suspected to cause it?