It looks to me like they had a thick strip of green fondant and aligned thin blue strips over the green fondant and rolled the blue stripes into the green with a rolling pin, then just cut the ribbon's sides to have straight edges
That ribbon is beautiful
You place thin strips of each color side by side and roll it out. I usually roll out a solid color then place the strips on top of this and roll it out. It gives you a solid base so the strips don't fall apart.
That is really pretty! I especially like how the edges are un-even.