Nope, that would be perfect. What do they eat in the wild? Grasses. We feed veggies and other stuff when we have a tort who wont eat that or dont ahve access to them.
My two large torts are outside in a pen with grass and i dont give them much of anything extra. Hard not to sometimes! so they do get treats or something for variety. but they have it out there. lots of diff grasses. and the poop to prove it!
Unless someone has done a scientific study on this, we can only give our "educated guesses". It seems to me that a diet made up of only one species of grass and nothing else, would lead to deficiencies over time. I think most people would agree that a varied diet would be better. However, you don't have to use store bought greens. You can use weeds. They are free, everywhere, and in my opinion, better. There are lots of sites that will tell you which ones are safe and which ones to avoid.
Having said that, I have heard of a large scale sulcatta breeder that supposedly feeds only Timothy Hay and nothing else, ever. I would love to have this confirmed or not. Personally, I wouldn't do it that way.