This might be correlation rather than causation. Perhaps Alzheimer’s Disease is associated with mental changes earlier in life that reduce the desire/need for Viagra. In that case, staying sexually active with a partner later in life and using Viagra is a sign that one is less likely to develop Alzheimer’s.
This is true, but the vascular nature of both does at least suggest causality; if there's any chance of making some progress on this appalling disease then that must be pursued.