They want you to machine stitch the binding down. They send enough backing that you have plenty to work with and bring all 4 sides to the top and trim and stitch down.

I've made my own QFK'S bindings also but I've hand stitched the binding to the backs. I asked and was given the OK. However, I use very small ladder stitches and secure every couple of inches or so and definitely at the corners. The way I bind the back is slow, but it's the process I enjoy the most. It slows me down in a good way.

This will sound odd but I've found I can manage best with a #10 Roxanne's Betweens needle. "Very" small needle but I manage quite well.