People with vision loss can use magnification, screen readers, or the combination of the two. People with dexterity issues might use a mouse, keyboard, or alternative keyboard.
You can’t tell anything from a person’s diagnosis about how they will interact with a website. Therefore, the focus should always be on the assistive technology. When you focus only on diagnosis, you are missing the fact that people can have more than one disability and those disabilities intersect.
Focusing on the medical diagnosis is following the medical model of disability. Focusing on access needs is following the social model of disability. Focus on access needs, always.
Alt: There is a reason W3C and WCAG don’t mention disability in any of their standards.
All product accessibility efforts must focus on assistive technology, and not the medical conditions people with disabilities have.