Seriously, how does “high blood pressure” REDUCE blood flow? It cannot. However, if the arteries are NARROWING due to something else, then high blood pressure is the body’s compensation trying to keep the blood quantity going.
Thus blood pressure is a symptom of something that can cause dementia, but reducing blood pressure by drugs would only FURTHER reduce the amount of blood getting through, making dementia more likely.
I think you are reading this wrong.
It is true that high blood pressure can result from restrictions in our body. In this case, they were noting that, because white matter has fewer blood vessels/capillaries, what high blood pressure you have (for any reason—even drug or hormone induced), hurts white matter the most.
In this case, the blood is getting there, but because the vessels/capillaries are few, it impacts the white matter more because there is less diffusion of pressure. Having more available vessels and capillaries helps to spread the pressure problem across more area. With less area to expand, what there is is more impacted. We want our vessels and capillaries to be plentiful to deliver nutrients, but these do clog up, over time.
You know your physics. Medical Doctors are not good in physics, thermo-dynamucs or fluid mechanics.
And they keep defining high blood pressure down, in order to get more people on the drugs. At one time 140/90 was considered normal.
This is a good video on the subject.
https://www.youtube.com/watch?v=fPsOX3S3mCE
“Seriously, how does “high blood pressure” REDUCE blood flow? It cannot.”
Not immediately, in “realtime”. But over time can contribute to plaques which cause friction and narrowing and hence reduce blood flow.