Actual study: https://jamanetwork.com/journals/jama/article-abstract/28447...

”After adjusting for potential confounders and pooling results across cohorts, higher caffeinated coffee intake was significantly associated with lower dementia risk (141 vs 330 cases per 100 000 person-years comparing the fourth [highest] quartile of consumption with the first [lowest] quartile; hazard ratio, 0.82 [95% CI, 0.76 to 0.89]) and lower prevalence of subjective cognitive decline (7.8% vs 9.5%, respectively; prevalence ratio, 0.85 [95% CI, 0.78 to 0.93]).”

So about 18% relative reduction. But if your risks are already low (e.g. active and healthy diet) the relative reduction is less impactful (e.g. 4% to 3.28%).