its hard to see the detail because the pic is small, but a couple guesses... piped royal icing to match the color of the bow or using an extruder to make a rope, then attached with gumpaste glue, or (cant think of the tool name= by wilton,a small "pizza cutter" style tool that has different designed wheels, you roll it and it impresses a design.
lovely cake!
I am also thinking a fondant cutter tool (the rolling wheel)... put a ruler on your edge to keep it straight and don't push all the way thru. it is a great look, you are right!
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