[dynisor]

I read it differently. It sounds to me like it’s saying that it was people “who weren’t able to see well when they were using their prescription lenses.” As in, they have prescription lenses, but their vision is not actually corrected.

[JeremyNT]

I agree, and to me it hints at a couple of possibilities...

Perhaps they are receiving sub-standard vision care (and are thus unable to get updated eyeglass prescriptions, cataract surgery, etc), and this might be associated with other forms of sub-standard care.

Or perhaps a person suffering from dementia is less likely to notice and/or less able to pursue treatment for their own declining vision...