I think the message is not as discouraging as it may sound at first!
The study had 3 groups of APOE risk (high - at least one ε4, intermediate - ε3ε3, low risk - at least one ε2) with 1847, 3717, and 887 people, respectively. Each group was stratified according to lifestyle: favorable, intermittent, unfavorable.
In the APOE4 group, there was no difference in dementia cases between the favorable and unfavorable lifestyle groups; while in the 3/3 group there was about 30% decrease for the favorable lifestyle.
But how was the lifestyle defined?
We adapted six health and/or lifestyle factors shown to be important during later life to lower dementia risk, as set out by a recent meta-analysis and endorsed by the World Health Organization. Among these are: (1) abstaining from smoking, (2) absence of depression, (3) absence of diabetes, (4) regular physical activity, (5) avoiding social isolation and (6) adherence to a healthy diet, which included limited alcohol consumption. Using these six factors, we computed an overall profile of modifiable risk factors with a score ranging from zero to six, and we subsequently grouped participants into three categories of modifiable risk (unfavorable, ≤2 protective factors; intermediate, 3–4; and favorable, ≥5).
Physical activity was defined as 40 minutes per week with a MET intensity of ≥4. As for a healthy diet:
To have a healthy diet, a participant needed to adhere to at least half of the following dietary guidelines: consumption of a sufficient amount of fruits, nuts, vegetables, whole grains, fish and dairy products, and a limited amount of refined grains, processed meats, unprocessed red meats, sugar-sweetened beverages, trans fats, sodium and alcohol.
These criteria are not very specific. Under the criteria, 2/3 of all studied people across all different APOE groups were in the "favorable" lifestyle group (at most one risk factor). It is remarkable that for ε3ε3 and ε2 groups, these "easy" lifestyle choices were already enough to produce an association with a decreased dementia risk. It appears that these "easy" lifestyle choices are not enough for us in the ε4 group, but maybe if "harder" lifestyle choices were studied (more exercise, even healthier diet, incorporation of fasting, etc) there would be an association with less dementia?
Thus, we need not be discouraged - we need to work harder on our lifestyles!
Could be the people in the study weren't doing a 12-16 hour fast most days, switching from bad oils to good oils, taking the supplements recommended by Bredesen or some of the others, etc., etc. 
