By that logic, is it silly to say that the root issue of someone's heart stopping was the bullet that passed through it, because a knife through the heart was the root issue that caused someone else's heart to stop?

There's different ways to get dementia. Alcohol consumption is the root cause of one of them, as well as 61 other diseases. If you get dementia and they run tests, they might find that the root cause of your dementia is alcohol consumption. They might find it's a different toxic substance you were exposed to, or they might find it was genetic. That's the root cause of your dementia. The study and article obviously aren't saying that alcohol is the root cause of everyone's dementia, or the 61 other diseases it causes.

It is saying that someone who wouldn't have otherwise dealt with dementia or 61 other diseases could get them purely due to alcohol consumption, making it the root cause of their disease.