I have memory loss issues due to seizures and severe Complex Post Traumatic Stress Disorder. I don't just forget events and pieces of information, but I lose skills (knit, divide, cook a steak), I am often very very literal, and struggle to understand context and the big picture.
Having worked with people with Alzheimers and suffering from some of the same symptoms, I would definately say yes, their disability would affect their ability to read. Reading is more than psychic ability and/or memorizing the meaning of individual cards. There is the need for application and context.
I struggle to read the cards more and more as I am deteriorating. I don't read for others anymore. I continue to self-soothe with my cards and just do what I can. Not that I was ever a good reader :-0 But still...I have noticed a significant decrease in my reading abilities.