The triangle is from the muzzle end, down vertical to the ground, then on the x axis out to the impact zone, then the h axis back to the window. If he is laying back on the carpet, he cannot see the impact zone. He has to protrude the barrel out the window to get on target.
Line of sight from the hotel to the concert is 1300 feet horizontally and 320 feet vertically - or a 4 to 1 ratio. Inside the hotel room it is 36 inches horizontally from the muzzle to the window and 9 inches vertically from the muzzle to the carpet - or a 4 to 1 ratio. Try to picture it in your mind - two triangles both with the same ratios of horizontal to vertical legs - now do you think the third legs of the triangles (the hypotenuses) would be at the same angle?