This sentence in the discussion gets to exactly what I was thinking as I read the study:

“Moreover, as in most preventive trials, our population resulted in being, through self-selection, enriched with healthy participants with high education and reserve, and low probability of cognitive decline, which left limited room for lifestyle changes and intervention-related improvements.”

The authors were using this to explain why they might not have seen as big an improvement as they thought they might find. I think it also suggests why it might not be very applicable to a broader range of elderly individuals at risk for dementia: They would likely lack sufficient motivation to stick with the program.