I think I've read the article differently to you. It does not say that the afterimage is a test. In fact it calls it an illusion.
> This afterimage illusion, while a product of perception, serves as a bridge to understanding visualization
The way that illusion works is simple. You expose your eye to green light without moving your gaze (stare at the dot). The green receptors in your eye become tired in the receptor pattern of the apple. Then, you switch to the white light which is broad spectrum. You fail to see the green, leaving red (blue is almost green, and our colour triangle is not equilateral). So, you see an afterimage of the apple but in red. This fades quickly as your green receptors recover from the overload. It has nothing to with aphantasia and the article does not claim it does.
> This afterimage illusion, while a product of perception, serves as a bridge to understanding visualization
The way that illusion works is simple. You expose your eye to green light without moving your gaze (stare at the dot). The green receptors in your eye become tired in the receptor pattern of the apple. Then, you switch to the white light which is broad spectrum. You fail to see the green, leaving red (blue is almost green, and our colour triangle is not equilateral). So, you see an afterimage of the apple but in red. This fades quickly as your green receptors recover from the overload. It has nothing to with aphantasia and the article does not claim it does.