According to the image, the electrodes reside in the vision related brain region. For instance, it is common that people can become blind, possibly permanently, if they get a hard blow to the back of the head. It can happen even, if people fall on their back and hit their head to a sidewalk edge or something.

So, the point is taht it might be that they do not have access to thoughst, but just the images that the person looks at.