There have been studies showing that higher coffee consumption correlates to slower cognitive decline. This study only looks at the effects of coffee in vitro.

You are right to call this out. Tau are protective, and if you have the genetics, coffee will raise the risk of AD.

Oxidative stress is the fundamental issue causing Alzheimer's. So there are many causes with one result.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5562667/

"Oxidative stress participates in the development of AD by promoting Aβ deposition, tau hyperphosphorylation, and the subsequent loss of synapses and neurons. The relationship between oxidative stress and AD suggests that oxidative stress is an essential part of the pathological process, and antioxidants may be useful for AD treatment."