Indeed, correlations are not sufficient to establish cause. However... things that do cause a specific effect also tend to correlate. So correlations are a basi

Indeed, correlations are not sufficient to establish cause. However... things that do cause a specific effect also tend to correlate. So correlations are a basis for hypotheses that can be experimentally tested. To wit, the hypothesis presented in the post linked to by OP can be tested by treating people with Alzheimer's disease with antimicrobial agents. Unfortunately, in what concern people (disease and dying) experiments on humans (aka clinical trials) are expensive, with uncertain returns. So the experiments that could test hypotheses concerning Alzheimer's disease in real actual people are rarely performed to the level statistically needed. Instead we get a plethora of studies in mice with "suggestive"implications for what might occur in humans. Along with random studies that are not worth more than anecdotes (see those I commented on above).