If that was the case then all people with plaques would have Alzheimer's, which is not the case. In my own lab I see brains from dementia free patients in their late 70s-90s who have plaques. It's an age related phenomenon to have brain plaques as we age. Additionally there are a significant nunber of AD patients who have all the classic symptoms of AD while alive, but are found to have brains that are completely plaque free during a post-mortem analysis.