From what I recall in my neuroscience class, this is likely to be caused by damage to the visual cortex, which is used to consciously process visual information, but not the superior colliculi, which is used to unconsciously process peripheral vision. This is caused by damage to one of the visual processing regions but not others.
From the article, it appears something similar has happened. Corticobasal syndrome potentially killed off some brain cells but not others, leading to damage at some areas but not others.
"Sara Ajina, a neuroscientist at the University of Oxford who studies visual awareness deficits such as blindsight—a residual, unconscious “sight” in people with damage to the brain’s visual system..."
From the article, it appears something similar has happened. Corticobasal syndrome potentially killed off some brain cells but not others, leading to damage at some areas but not others.