If a lower level rider is only practicing the reinback in and out of the walk in the home arena, I think it’s more than fair for GP riders to point out that they have no appreciation of what is required of a GP horse. A GP horse is ridden to be prepared and available at every single stride to do whatever the rider asks, whether that be to maintain, extend, collect, transition, move in any of a variety of lateral directions, etc. And at an exact letter, not “whenever the rider feels ready.”
The lower level horse in this example reins back then walks forward. Period. That’s all the poster does.
The GP horse reins back a precise number of steps and then may be asked to canter, piaffe, passage, extended trot, stand immobile, collected trot, extended walk, etc. I school all of these variations. A GP rider aims to have the horse balanced, active, prepared, and listening at all times to make any required change or transition at any time while maintaining the quality of the gaits throughout. Now add the pressure of a show environment and that reinback may not happen as well as it does back home.
BTW: What is the most important part of the reinback? It’s that first step forward when the energy coiled in the hind leg is released forward.