Agree on the principle but a disagreement on a point of pedantry. He said a blast radius of 8 km or 5 miles. The distance is from the centre of the explosion or the dispersion to the edge.
8 km wide would mean a diameter of 8 km or 5 miles.
The difference is a factor of 4.
If 8 km is the radius then the area is about 200 km2 or 200 grid squares.
If 8 km is the diameter then the area is about 50 km2 or 50 grid squares.
Either way, 20 mortar rounds in 50 or 200 grid squares, they are going to need to pump an awful lot of rockets into that target area to have any kind of militarily useful effect.