I emailed Fox to ask about the 2.5 rear on an AT4/TB, if anyone else is interested here’s the reply:
Since you are running the 883-06-157 up front which will maintain the factory ride height of the AT4 in front- no ride height increase, you can use the 883-26-060 in the rear to complete the ride dynamics upgrade.
Since the AT4 and Trailboss are already 1.5" taller in the back than the standard trim levels, the note in regards to the rears is to prevent AT4 / Trailboss owners from increasing the height in the back when using the 883-06-162 up front.