I think that the rope was at the location before the crime. Trash, detritus of farm life. Whether it was used by the offender at any point in the crime, I don't know, but I think the DT theory of the girls being strung up is wildly speculative and doesn't fit the little we know of the physical evidence.
The other thing I do know is that there's no sign in the released stills of BG that makes me believe he's carrying a length of rope long enough to restrain both girls.
So if it was used at any point, I think it was a found object that was useful, not something he brought to the crime scene.
If the girls were bound tightly enough to restrain them at any point, then I would expect there to be evidence. Bruising, fibre transfer, etc.
MOO