There were those studies that showed moderate alcohol use improved health and only heavy drinkers saw detrimental health effects. The problem was that "no drinking" group included people that weren't drinking because of poor health. Later studies compared drinking vs. a "no drinking" sample of people that drank around two glasses of wine per year. The improved health effects completely disappeared. The more you drink, the worse it is for your health.
So this study is like that in using a potentially unhealthy comparison group. They try to offset that a little by also throwing in people that quit drinking. But it is likely that some people quit drinking because of health problems. So I'd guess that this study has the same problem with an unhealthy comparison group. The study probably can't tell you what the actual relationship between alcohol use and dementia with any authority.