From what I understand, the medicine available is fairly successful at staving off further disease progression but the normal late stage diagnosis often means it’s far too late. This might make outcomes significantly better if it works as well as reported.
That is true for some diseases but not all. I don't think we know how to prevent dementia. But identifying biomarkers is useful, at the very least for selecting people for enrollment in future trials.
The article states there are medications that can slow or reverse the disease, if the disease is diagnosed early enough.
"An early diagnosis is critical for those with dementia. New drug technology can slow, or even reverse the progress of Alzheimer’s, but only if the disease is detected early enough. The drug lecanemab is one of two new treatments for the disease."
That's like weight-lifting to prevent ALS. Dementia has multiple (suspected) causes, and they are unrelated to cognitive activity. There's no evidence you can prevent plaques or cardio-vascular damage by reading or puzzling, AFAIK.