I know that words for mental illnesses have changed quite a bit in the past century or so. Informally, I think most people see a difference between "crazy" and "unintelligent" today. "Dementia" is also understood to mean a general loss of mental ability: memory and cognition. But in the 1920s, schizophrenia was called Dementia Praecox. Did the medical term reflect a different understanding of schizophrenia, that it was a sort of what in modern times would indeed be called dementia or did it reflect a less strong distinction among medical professionals a century ago between "crazy" and "unintelligent." I think it is true that in schizophrenia there is indeed a cognitive decline but I do not think that is the primary characteristic and also I believe schizophrenia sometimes goes away where as what is today called dementia almost never reverses and in fact usually continues to get worse.
What got me going on this is I was reading The Making of the Atomic Bomb (Rhodes) in which Oppenheimer in his 20s was diagnosed/misdiagnosed with "Dementia Praecox" -- certainly there are few men less likely to have major cognitive deficits. He was in the middle of getting a doctorate in physics and he would end up managing the technical aspects of the Manhattan Project, etc. -- he was a well-known smart guy. So I remain confused about the term.
However, while J. Robert Oppenheimer certainly was a great genius, capable in many different areas, mathematics, physics and even the humanities, he did behave strangely in his 20s, attacking one friend and trying, literally, to poison another.
So to me he sounds "crazy" (in the normal sense of the word) but not suffering from cognitive decline.