Alzheimers has a very long development stage compared to almost all other dementia (Lewis body also has a long timeline, but none of the other forms has anywhere close to the known length for Alzheimer’s disease). The original study article has no causal analysis, so a simple explanation is that super early Alzheimers symptoms (possibly even undetectable ones outside the context of intense taxi driving) drives people to abandon their taxi- or ambulance-driving jobs and seek other forms of employment.