Elles that is exactly what your first post says, that they jump through the brushes… If you look at any video of steeplechase, you will see them jump through the brushes.
As for your Maryland Hunt Cup, even if their is a 4’10 vertical over a 22 fences course, it only means that some of the horses can jump 1 fence of that hight. Many many horses can do that and I agree, you will find many TB able to jump one fence of that hight. That doesn’t mean they could jump an oxer, or a combination, or the same fense from a tricky distance, or even 4 in a row. And the speed helps. I am not going to give a crash course in physics here, but to jump a fence, you need to adapt you trajectory taking into account the with and the hight you need to cross. Then it is all a question of energy. Your speed will allow you to cross the distance so your force can be applied to your push up. To cross same fence from closer, at a collected canter will imply that your horse will have to generate more energy on the push to cross bot the width and the hight. I am no physicians and I am sure someone better than me will correct me on the specifics but I think that this is generaly correct (or at least reasonnable form someone who studied physics in highscholl 3 or 20 years ago…) . But when you learned high jump in school, you learned how to generate energiy in your approach, and how to transer it horizontaly so you only have to push up and bascule. Same with horses.
