Aside from alcohol and obesity, I don't see: 1. Cancer survivability. If people aren't dying from cancer they will be more likely to die from a fall in a weakened state. 2. Two level homes are a higher percentage of new construction and people are more able to afford it when they're older.
Edit: okay they did mention fewer deaths from other causes such as cancer and heart disease.
The article did address differences in the rates of 'same level falls' vs. stairs, etc. So they accounted for that too.
We just finally got my 85-yr-old mother to move out of her house into a nice apartment near us with elevator access. She has already had some bad falls. Now we just have to worry about her driving. She shouldn't need to drive as much in her new place, but she probably will anyway.
Fewer small (one-level) homes is an excellent point. Also a lot of old people used to live in trailer homes or mobile homes, which are also one-level.