Close, but no cigar.
Essentially, the danger zone is the area where a round would strike a standing soldier. The area between where the trajectory of the rounds brings them down to head height, and the edge of the beaten zone closest to the gun.
Thus this is the area between "first strike" and "first graze"
The diagram that you've been given is a misleading oversimplification. The trajectory of the rounds is never actually flat. From the moment they leave the barrel, gravity is acting on them. They begin to drop as soon as they leave the barrel. The gunner angles the gun upward to counteract this, and allow the rounds to strike the intended point of aim 800 meters downrange. At longer ranges, this "arc" becomes more pronounced. The danger zone only begins when this "arc" approaches to within man-height of the ground.
Thanks a lot for the whole explanation, i guess i was oversimplifically mislead then