The study clearly demonstrates that what one eats can greatly influence the risk of dementia, like also for many other old-age diseases, but its scope was far too limited to allow the identification of the food components that determine the risk of dementia.
Others have said that perhaps not dairy fat per se was beneficial, but some fat-soluble vitamins and essential nutrients.
It is also possible that those with higher dementia risk did not eat enough fatty essential nutrients, but this deficiency could have equally been covered by other healthier sources of fat, e.g. by a suitable combination of vegetable oils, but the study has not investigated at all this possibility.
Anecdotally, due to my personal experience, I believe that dairy fat is harmful for cardiovascular health.
About 5 years ago, I became scared when a medical checkup had concluded that I have symptoms of incipient atherosclerosis.
I examined what I was eating, to find what I could improve. I was already eating mostly healthy food and in an amount small enough to maintain constant an optimum weight. A couple of decades earlier, I had been obese for many years, but then, after some failed attempts, I had eventually learned to control my weight and I had reduced my weight to 2/3 of the previous weight, and then I have maintained that weight until today.
The only wrong thing that I could identify in my diet was that I was eating very large amounts of dairy, thus a high amount of saturated fat. I stopped immediately eating any dairy. Since then until now, more than 90% of my daily intake of fat is provided by a mixture of oils that I use in cooking my food, which has an optimum profile of fatty acids, according to the current knowledge, i.e. the most abundant fatty acid is oleic acid (provided by olive oil, avocado oil or high-oleic sunflower oil), there is an adequate amount of linoleic acid and vitamin E (provided by cold-pressed sunflower oil) and an adequate amount of omega-3 DHA & EPA (provided by cod liver oil), to which a drop of an oil with vitamins D3 & K2 is added.
After a year since the switching from dairy consumption to consuming a good mixture of oils, not only the symptoms of incipient atherosclerosis had completely disappeared, but also other signs of bad peripheral circulation that had affected me for many years, e.g. cold feet, had also completely disappeared.
Therefore, based on the success of this change in my diet, I believe that my initial assumption, that the fat from dairy was harmful for me, has been correct. Today, the only products derived from milk that I continue to eat are whey protein concentrate or, more rarely, milk protein concentrate, which have negligible fat.
Also in the medical literature, there have been a great number of studies where replacing dairy with better sources of fat has lead to improvements in cardiovascular diseases or in some other diseases, e.g. multiple sclerosis.
In conclusion, I believe that this study is correct in the sense that an adequate intake of fatty nutrients is important for the health of the brain and of the nervous system, but it has not demonstrated at all that dairy fat is the correct solution, because dairy fat has not been compared with other sources of fat.