It's interesting that they were not able to extract good images from the test when they just asked the subjects to imagine the pictures. I speculate that is because the vast majority of people are able to recognize images of things, but not produce those images.
For a simple example, see this article about people trying to draw a bicycle. Before clicking, try to draw a 2D representation of a bicycle -- the wheels, the frame, pedals, handlebars, etc. You'd be surprised how difficult it is. https://road.cc/content/blog/90885-science-cycology-can-you-...
I would be very interested to see them repeat the experiment on e.g. artists who specialize in photorealism, or asking an expert on a very particular thing to imagine that particular thing (e.g. ask a violinist to imagine a violin). I wonder if that would yield more recognizable results than asking a random grad student to visualize a lion's face.