With a lot of genetic diseases, lifestyle choices can definitely contribute to how early they're contracted, or whether they're contracted.

The reason it wasn't mentioned is because they weren't studying the mechanism of impact. The goal here was just showing the link between being vaxxed for other things, and having a lower risk for all forms of dementia; it wasn't to show why vaccines lower the rates. My guess is lower inflammation rates, since inflammation does a lot to us

Edit: The next comment down (at the time of writing) also shows a link between inflammation/viral infections, and dementia

Most ALS is not genetic, and im not sure about parkinsons, but I suspect it's similar. Lots of studies these days are finding links between viruses that dont actively cause much harm when you are infected, but and up being highly associated with autoimmune diseases. A prime example is Epstein-barr virus and MS. I have no clue what the mechanisms are, and im sure scientists are still learning, but basically an infection could cause an immune response that is too strong, and even after the infection is gone, the response remains.