Indeed - it could equally be explained by dementia/Alzheimer’s being a contagious disease, such as a prion or similar protein malformation, which quite a bit of current research seems to suggest, and it simply had not become endemic at that point.
If it were a prison disease, surely with ~2 billion data points of those with/without dementia, we would have isolated exactly what molecule causes it and how it gets transmitted from person to person.
Track the incentives.