Direct link to reference 7, "Familial and sporadic fatal insomnia": http://a.pomf.se/7Uu7.pdf

From the paper, "Phenotypic variability is another issue. In some patients, insomnia, one of the cardinal symptoms, was not reported."

The other half of that sentence is obviously "... but Dementia and death still occurred."

Therefore, this research demonstrates that fatal familial insomnia results in Dementia and death, but with insomnia as a secondary effect. The insomnia is a symptom, not the cause.

I'm only trying to show how tricky it is to base an argument on scientific research. There are all sorts of corner cases and gotchas. In this case, the fact that insomnia is unrelated to the Dementia and death is buried on page 8 of a 10-page article, embedded in a gigantic paragraph. A rather important point for such little treatment by the article!