It’s not. The Pro’s screen is an extra 28 pixels bigger in each direction (784 pixels total) compared to the screen on the Air. So the Air does have slightly thicker bezels.
Seriously though, what a weird arbitrary differentiator between the 2 devices. 28 pixels difference in each direction - why even bother at that point other than to have a difference on paper? If screen size is a big factor in your decision on which iPad to get, don’t worry about it. No way you will miss a measly 784 pixels on a screen with 3.8 million pixels. The Air and Pro screens are essentially identical in size.
I suspect that Apple probably did consider a 10.8” screen at some point as the rumors indicated in order to have a more real difference in screen size compared to the Pro. But, I’m betting the bezels looked so big that they would have sounded silly saying it was an “all screen design”. So, they were forced to go with a 10.9” panel instead.