That is close to what I am saying. We know BG crossed to bridge up to a certain point. What he did after that point in the video, we don't know...until he said "down the hill," and at some point later killed the girls.
LE has told us the girls crossed the creek, so we know that. I'm also not sure where Libby's father was set to pick them up. I guess I've assumed at the same spot the sister dropped them off. I think if they had crossed the bridge, turned straight around and gone back the same way, maybe the timing was right? Not sure.
But at the moment, I'm putting a lot of thought into him cutting them off from the north after they crossed the creek. It does make more sense from BG's perspective than he himself crossing the creek. I am probably thinking beyond rational, though. It doesn't feel like a popular theory.