I didn’t take your comment as bashing any breed
This is where it gets unfair. Not every horse is capable of correct GP, no matter how talented the rider, and that applies across every breed, and applies to most horses in a lot of breeds. So no, no single GP rider is capable of taking a functionally downhill, butt-high, sickle-hocked horse (of whatever breed) to correct GP. He’ll break first, and will be physically incapable of some of the required movements.
Most competent GP rider should absolutely be able to bring any well-enough conformed horse, with the brains for it, up to GP competence.
But there are some who require btdt horses to compete on, not having the fundamental education on how to train horses up the levels, only how to ride what’s already there, learning the buttons to push to get the pre-installed movement. And absolutely they exist, they exist in every discipline.
Not everyone has what it takes to be a teacher, for a variety of reasons.