The PCA actually has good details about what is on Libby's video.
It shows Abby walking southeast (so toward the south end) and it shows a man in a dark coat and jeans walking behind her.
Then one of the girls mentions gun
The man is SEEN and heard telling them down the hill
They then proceed to head down the hill and then the video ends
So BG is not only seen behind Abby at the beginning of the video, but he is seen and heard when he's instructing them to go down the hill. I'd say maybe his face isn't visible then because he was so close to them that maybe where her phone was at only showed his body or midsection or something like that, but based on the timing LE can say it was the man approaching them that is then seen and heard telling them where to go and they go that way.