This is the answer that the support team gave me:
We normally get the reverse question and here is the response:
The mounting bracket for the iPad Air 2 (not original iPad Air) might physically fit the iPad Pro 9.7" but the camera/sensor alignment will be off and Calibrator will not be able to properly compensate for it. We do not recommend this configuration and offer an Accessory Bundle specifically designed for both iPad Pro models. If you attempt to fit your iPad Air 2 bracket over the iPad Pro 9.7" anyway please be very careful because the iSight camera protrudes slightly from the main body and there is a risk of scratching the camera lens and/or damaging the bracket. Also, if you have a Wide-Angle Lens it will almost certainly scratch the camera lens if attached to the mounting bracket because of the protrusion.
The inverse of the above statement implies that the brackets designed for the iPad Pro 9.7" might physically for the iPad Air 2 but the camera/sensor alignment will be off and Calibrator will not be able to properly compensate for it. The issue regarding the protruding iSight camera is, probably, not relevant.
In other words, it would fit but it may not be possible to achieve proper calibration.