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.
Yes, clogged blood vessels are A - not the, but a - cause of high blood pressure. Are they the only cause? If not, then do the other causes result in any less blood arriving on site?
For example, if the main aorta does not expand, the systolic blood pressure will be high while the diastolic blood pressure remains unchanged. This has no impact on blood flow anywhere else. So...how does that impact capillaries?
And how does treatment of high blood pressure affect blood flow? If your problem is clogged arteries, and you reduce blood pressure, it requires LESS BLOOD to flow thru the artery. Pressure times diameter equals flow. How does less blood arriving help with starved cells?
Have you then treated the problem, or masked it - and by masking it, made it worse?