I think theres little doubt that Gal uses Rolkurr but our UK riders are under extreme scrutiny because the practice gets such bad press there that I doubt Karl Hester would support it
That doesn't mean to say he doesn't use flexion exercises that involve having a horse behind the vertical for short periods of time - the difference is how long you hold it there, how much strain you are putting on the horse to get it there and how close to the chest you have the horse working
A forward going horse that's supple and has lots of impulsion and a really light mouth is actually very easy to get behind the vertical.
You can see the difference in a horse like that because its still moving fluidly and looks relaxed as opposed to one that's forced, stressed and stilted
This is Dances with Wolves at one of his early shows at Hartpury, https://www.youtube.com/watch?v=SwYnAJD3LF0
Uthopia seems to perform better for Charlotte than he did for Karl which might say something about her style of riding