I don't know, I work with horses a lot, and have for many years. This isn't surprising at all. They are intelligent. Like humans, they have a variety of intelligence, a spectrum. I think that any human that has spent time with them, and has the ability to reflect, knows this. That's why they challenge us and also why they can be such great resources.
Agree, horses can be very intelligent. Good cowhorses are prized for the way they think and if they can outsmart cows, along with their athletic ability, so this doesn't surprise me at all either. I also think horses have a sense of humor and like little pranks.