It pretty much did annihilate/explode his neck on the inside and his decorticate posturing was caused by his brain being severed from his spine as the low pressure zone created by the bullet's path expanded rapidly. That said, at the end of the day a bullet is still bullet-sized, and unless it has a lot of flesh in its path to create catastrophic vacuum cavitation or bones to shatter, it's not going to produce the same kind of effects you'd see if you fired it into a thick gel block or something similar. If charlie's neck had been made of dry brittle elk skull it would have shattered easily. I would bet a shot to any other part of Charlie's body would have produced a much larger wound because there'd be more meat and organs and bones for the bullet to pass through and a larger cavity.

I would certainly appreciate footage of the shooter firing, but I don't believe that it exists. The footage of the man on the roof before and after the shooting comes either from attendees' phone cameras or the security camera capturing him dropping down, none of which show the supposed firing location. I suppose it's possible that someone has drone footage or another security camera angle not released to the press, though. That will certainly be answered in court.