'Ideal biomarker' detects Alzheimer's disease before the onset of symptoms

This sounds almost too good to be true. It's published in a lower impact journal so I'm wondering if the sample size was very small. Also, how did they confirm the 'topography of the pathophysiology' (where the disease is) without taking people's scalps off?

The article claims this test will promptly be used clinically. Diagnosing Alzheimer’s while there is no cure or treatment available is useless. If used, the test unnecessarily burdens patients sooner. I can see the importance of this test in helping to find the root cause of Alzheimer’s, but I can’t see why this test would be used clinically while we can’t do anything about Alzheimer’s.