Well,
Coach isn't telling us how we should their products. I'm sure the company appreciates when clients take what they give them and make it their own. I'm expecting to see quite a few fashion bloggers get unconventional with these sets.
However, it is a strap for the Duffle. Not a replacement strap, but an accessory. I foresee Coach marketing this as a quirky way to let clients customize their Legacy Duffles. I'm not sure what bag the strap fits, but I would guess that it is the same as on the Large Duffle.
I can even see the strap going really well with the BoHo Duffle. Now it can be colorblocked and worn cross body! I like how Coach is really making the Duffle it's "mascot" bag.