In this case, it appears to be the caffeine. Also, with these sorts of large studies, the amount is self-reported, so "a cup" is whatever the person reporting considers to be a cup, it's not some controlled amount.
Question: Is long-term intake of caffeinated and decaffeinated coffee associated with risk of dementia and cognitive outcomes?
Findings: In this prospective cohort study of 131 821 individuals from 2 cohorts with up to 43 years of follow-up, 11 033 dementia cases were documented. Higher caffeinated coffee intake was significantly associated with lower risk of dementia. Decaffeinated coffee intake was not significantly associated with dementia risk.
Meaning: Higher caffeinated coffee intake was associated with more favorable cognitive outcomes.