In the ad, a user holds up both devices side by side. The iPad's screen is almost completely unreadable and it's annotated with the message. "(Yes, the iPad is on.)" The user holding the devices says, "The iPad is heavier. It's really hard to see. Kindle I can see very clearly."
Amazon doesn't say what brightness level the iPad is set at; however, it seems as though it may have been turned all the way down for effect.
Although, this may be unfair comparison between different devices, Amazon's Kindle Fire HDX display was recently found to be superior to the iPad Air's.
Take a look at the ad below...