I board a classical dressage barn now (I previously lessoned at hunter barns for the longest time) so I'm learning a whole different way of riding now.
The video is not bad but I think it can be a little misleading to some people. Muscles only "work" in contraction... so building those topline muscles is only happening when those muscles are contracted. That is when the poll is up and haunches lowered and the abdominal muscles are pushing the back up into the rider's seat.
Working long and low all the time (which this video does not promote but because most of the clips are in long-and-low, some viewers maybe be misled) isn't going to build the muscles along the top of the neck and withers and over the haunches because those muscles will be "stretching" not contracting.
An exercise I was taught at our current barn is to raise your hands up high to "back" the horse onto his hind legs and lifting weight off the front. Your horse literally feels like s/he's grown another hand taller. Then ask for your transition to walk/trot/canter.
So you could ask for trot - halt - lift the shoulders - then trot on again on weighted hind legs. So, we're sort of doing backwards than in the video, where they start low then ask for high but theoretically ending with the same result.
I'm still learning a lot of this classical dressage from the barn owner but we've had much better results with it than any other place I've ridden. It certainly transfers over to other disciplines as well... several other riders also do horse-trials. Being light in front certainly helps on the cross country course and in the stadium jumping.
I don't believe in limiting yourself by being too rigid. I am not a dressage rider but I have always taught my horses to stretch. It seems to make them more pliable mentally and it does round their backs. I also raise my reins a little when I am asking a green horse to elevate and collect.
Stretching with a young horse can be elusive. My 6yo KMH will stretch nicely on a slack rein. My 6yo QH will evade the bit when he looks like he's stretching under saddle. I'm working towards using my bitting rig by lunging my QH with saddle and an egg butt snaffle bridle with lunging reins attached to the pommel, but the length creates a light contact. After I lunge him I can ride him with good contact bc he's warmed up and carrying himself better. I hope to get him on more of a flex after he shows me he's relaxed and obedient and will trot on the lunge for just as long as I ask. Right NOW I ahve to continue to drive him--that is months down the road.
You DON'T want your horse to nose and pull the reins away from you. That is not stretching. Going from collection interrupted with walking on a slack rein will produce what you want.