Given the age group (mean age of 71), I wonder if it's partly an effect of people with mild cognitive impairment, which the study explicitly mentions are includ

Given the age group (mean age of 71), I wonder if it's partly an effect of people with mild cognitive impairment, which the study explicitly mentions are included, being more sedentary (i.e., the effect is backwards or bidirectional). You could also imaging other risks of aging increasing both sedentary time and harming the brain (e.g., social isolation).