A woman who says she witnessed the shooting...
This is the first line in the linked article.
Then, later in the article, this:
Crenshaw said it took her about 30 seconds to run and get her phone, which she used to film Wilson standing over Brown’s body.
So, she did NOT witness the shooting...
SO it would seem that her video was after the fact.
The key question is whether she saw the shooting , and then went to get the camera.
The next question is : How credible is she ?