I’ve watched this clinic video series (3 parts) a couple times. As a dressage rider dabbling in low level eventing, I like a lot of what Jos says! I won’t claim to know what he means to say at that 54 min. segment, but based on his ESL verbiage, here’s my take on it. He warns of going to a strictly dressage trainer, and I think he means “upper level” trainer. And if you know of the competitive dressage trainers in Europe, there is a lot of rollkur and tight riding going on.
Good Dressage, schooling up to say 3rd level, is a good foundation for all horses - jumpers included. If you learn to control the front and hind end (shoulder-in, haunches in, HP and flying changes) you can control your horse well. Not relying on the gadgets, crazy bits, etc. G. Morris always warns about.
From watching Rolex dressage tests, which is essentially 3rd level dressage, that’s the highest level of dressage most of us will ever need for eventing competition. Jos also states that many talented and promising jumpers top out after 7 yrs due to being over-ridden by the hand, draw reins, hard bits, etc. Not because of dressage. Too much hand and overbending ruins the gaits and subsequently the horse’s jump. I know of many dressage-only horses who’s walk has been ruined (become lateral) by too much hand or over-deep riding. They also break down from not being able to use their bodies correctly after being kept in a false closed frame.
This is how I took his statements. Tho my first love is dressage, I was not offended by what he said as I agree that it’s difficult to find truly correct, dressage training.