To me, it does not matter one bit if Abby was standing or walking. Apparently, the grandmother of Libby verified this particular timeline to BH at least that is what he stated in his video.
We have always heard about a couple being seen by, at the very least, one witness. The video tells us that Derrick was told by a 70 year old-ish man that he didn't see the girls, only a couple under the bridge.
There is an hour and 27 minutes (approximately) from the 2:07pm Snapchat photo of Abby to when Derrick (as told by Becky) arrived at the bridge at 3:30pm and spoke to the 70-ish year old man. I don't think this is 'precision' timing for a disappearance.
Becky also told BH that Libby was off the SE end of the bridge when she took the image of BG.
LE has interviewed everyone they could find who admitted to being at the bridge/trail during the time in question.
BH is debunking theories and red herrings to narrow the discussion to known facts. Kudos to him for his effort.