All that cheery pro cycling stuff having been said, cyclists with the APOE4 gene did not see the same benefit. APOE4 is a known risk factor for Alzheimer’s. Researchers also acknowledged that while the study adjusted for age and education, its findings are purely observational. It’s not establishing a 100 percent definitive pattern of cause-and-effect.

Also important, is most people suffering from dementia don't recognize what's happening.

However, they'll still respond to symptoms by starting to avoid activities that give them difficulty. Even if they're not consciously understanding why they're not enjoying a hobby anymore.

So I wouldn't be surprised if further studies show that as a cyclist starts to develop dementia, the loss of ability in navigating their surroundings causes them to stop riding as much. If that's true, then that accounts for why cyclists would have lower rates of dementia...

Because the ones developing dementia, stop being cyclists.

Quick edit:

What would be really interesting is looking at rates for elderly cyclists who have switched to a stationary bike, and if symptoms were starting to manifest before or after the switch.