You're definitely not alone. It makes sense on an intuitive level. Even when you "close" your eyes, you can't stop yourself from seeing. You're simply staring at the inside of your eyelids.

EDIT: I had not heard of the Ganzfield effect before, although I was familiar with the phenomenon itself. It does seem like this a manifestation of what they're referring to. As a side note, in sensory deprivation situations you also become keenly aware of the sound of your heart beating and the sound of your blood circulating in your veins. It's very odd.