I immediately go to these two thoughts:
- Is significant life-long usage of real-time mental spatial navigation protective?
- Are those who end up in these positions self-selected for better than average real-time mental spatial navigation and that above average performance correlates with protection against Alzheimer's.
I think your 2nd point is less likely.
Anecdotal, but I've spoken with many taxi and ride-share drivers, and my impression is that their decision to seek out and continue that line of work is almost always driven by outside economic considerations. I've never heard someone base their decision on their ability to perform the job.