Easy to imagine vascular dementia rates being higher due to air pollution, would be interesting to see if rates of non-vascular dementia, such as Alzheimer's are also higher.
It's equally easy to imagine that people who have a brain chemistry that makes them value things like quicker access to transport or a faster pace of life that makes them want to live in the centre of a city are also more susceptible to dementia though. This research has highlighted an interesting correlation that's worth studying, it's not necessarily showing that pollution is the cause.