A great example of the below 50% situation is the picture of the avocado and the tomato. I was confident that that was AI generated because I was pretty sure I'd seen that specific picture used as an example of how good Dall-E 3 was at normal text. However, most people who had used other models were probably used to butchered text and expected that one to be real.
If they did this quiz again with only pictures that were sketches, I bet the standard deviation would be much smaller.