Val, absolutely you can learn on a green horse too, I have done that for most of my riding career. But as Faye also experienced, I learned so much more in one lesson on an FEI horse, than I have in months on a green horse. And when I went back to green horses after riding more experienced horses, I was able to get them progressing 10 fold faster than what I had before.
When you're trying to discover how to get a horse through the rein, on a horse that has no idea itself, it's **** hard, you don't know exactly when to put an aid on or take it off. But getting the feel on a more experienced horse, you can go back to your green horse and apply that concept to get a better result.
After getting a good feel on an experienced horse, you will be a far more effective rider and trainer as you know exactly what you are aiming for with your green horse, where as if you never ride an experienced horse, you are forever sort of floundering your way through training not exactly knowing or understanding where you need to be and what feel you need at that point in training.