Up to the mid-70s psychoanalysis was one of the core clinical practices in American psychiatry (the evidence is the shift between the DSM-II in 1974 and the DSM-III in 1980), and what psychoanalysis means by "schizophrenia" is something entirely different from what the modern approach means.
This was a wildly serious problem. Psychoanalysis was really only practiced at scale in the US and Argentina, so people made up theories of why New Yorkers were more psychotic than Londoners (this is somewhere in "Listening to Prozac").
I think what has happened in the mental health field (and I've been a psychiatric patient-consumer for 15 years) is that the DSM movement has finally fixed definitions in place. But defining something doesn't mean it exists (except as a cluster of symptoms that front-line clinicians treat out of "data science" type knowledge). It's possible that the only clear-cut differential diagnostic is the one made by Kraepelin in the 1890s between what's now called bipolar and "precocious dementia" (more or less schizophrenia, but who knows what schizophrenia IS rather than IS NOT). And even this is challenged in the literature. How could it not -- looking at an episode and not doing the elegant analyses made possible to Kraepelin from working in a psych ward full of unmedicated bipolars -- how could you distinguish manic and schizophrenic hallucinations?
My doctors have kept me reading the academic journal literature over the years (abeit literature about bipolar, not dissociative disorders); but by the same token I figure it's difficult to place someone's multiple-personality story in any one of the available boxes (dissociative disorders, together with amnesias and fugues; schizophrenia; malingering). What the bleep do we know.