The riders are not leaning back in that they are at all behind the motion of the movement. It is more a position to be able to apply the seat aids in a way in which they are effective during a test. The core is still very much engaged, infact the position feels very much like a sit up where the entire back has just left the floor and you are lifting your chest to meet your knees, except every stride of the movement.
Leaning back is a position fault as it implies the rider has fallen behind the motion of the horse and is not using the core to balance independently from the hands. A positioning of the body where the shoulders are behind the vertical can be useful and when done in balance is "correct", although not ideal.
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