Watch the part where he is on the ground yet still reaching for an object. If Van Dyke thought it was a gun, he walks, provided jury follows Graham v Connor instructions.
The "reasonable officer" standard of Graham v Conner is an objective standard, which means it won't rely entirely or even mostly on Van Dyke's personal opinion that he thought the victim was reaching for a gun. The standard is more like, "if any other reasonable officer would have done the same thing under the same circumstances," then he comes out OK. Based on the video, I don't see that working real well for the jury in this case.
Peace,
SR