Some studies find that "schizophrenia runs in lower average IQ families, autism runs in higher average IQ families", from which some suggest that the autism-vs-schizophrenia distinction is grounded in a higher-vs-lower IQ distinction – see e.g. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4927579/ – but I think that kind of research is very speculative and needs to be taken with a large grain of salt

Other researchers suggest the boundary between autism and schizophrenia is weaker than is commonly supposed. Common diagnostic tests for ASD perform poorly in distinguishing it from schizophrenia; there is heightened prevalence of each condition among those diagnosed with the other; autism started out as a new name for childhood-onset schizophrenia (in fact the word "autistic" was originally coined to describe schizophrenia), and there is still a lot of dispute about what is the difference between autism and childhood-onset schizophrenia (the former is much more common than the later–subclinical hallucinations are common in children, but clinically significant psychotic symptoms are quite rare)

Nobody really knows, not even the experts.

there is a theory along these lines. something like: the defense against the unconscious is so strong in autism that the person becomes rigid and shut off. the defense against the unconscious is so weak in schizophrenia that the person is overwhelmed with phenomena. but both have something to do with intense unconscious objects.

Some scientists suggest that schizophrenia is the human equivalent of domestication syndrome. Autism is also seen as a type of anti/counter domestication syndrome.

Just gotta find the perfect balance between the two