I'm assuming you mean that Pre-fired perfectly timed OSMAW Shot is proof hes walling?
That little alcove is a great place to post up with just about any weapon, let alone an Osmaw. He very likely could of seen you coming up, as you were in his line of sight (remember 3rd person camera) for approx 4 seconds. From there its just some basic prediction, and remember, its possible that was a really lucky shot.
So no, Nothing in that clip is proof.
Also remember, because APB, his character was facing one way, but he was potentially looking elsewhere, so when he did start his wind up it gives the appearance of snapping.