It's most likely highly correlated, but not causative. That correlation still makes it diagnostically useful, which is how it's used here.
The most likely theory I've seen is amyloid causes tau buildup which leads to the majority of the damage. And by the time they give anti-amyloid drugs it's too late and the tau is doing the majority of the damage.
An analogy would be if you have a hole in your house which leads to getting a rat infestation. Once you notice the rat infestation patching the hole isn't going to solve your problem. The hole caused you to get rat shit on your counter, but by the time you fix the hole it's too late to stop the rats from shitting on your counter.
Neither of those are particularly easy procedures, but certainly better than a biopsy (and autopsy!)