The boulders are real!
I think of the stones as pinning the steel in place since the walls are all slightly concave and as the steel flexes in the middle it will push the stones sideways. These stones are quite well situated for any sideways forces.
Since this was the first time we did this, and couldn't find a single example, story, or photo of it being done anywhere else previously, we figured we would just find a way and make it happen.
Here are photos of the process.
We subgraded the terraces and placed the carefully selected stones. Then with a piece of pvc pipe, a laser level and a bit of trigonometry, we went to work on the stones and cut the back and bottom sides of them away. Then we sent off the specs to the steel shop and crossed our fingers. The steel came and a crane truck helped us put them in place. What do you know, they fit!
We can do it much quicker now with way less stone cutting, and we still use the pipe, laser level and trigonometry.

Those walls and how they fit into the rocks is truly a beautiful thing!! You nailed it on the head!!

so awesome! Hot or cold rolled steel?

Did you do anything to treat the side of the steel that is against the soil? Seems like in the NW, this would rust out?

Did you use Cor-Ten? Why or why not?