The original source was written by a journalist with (presumably) no military, science or engineering background, so "one tonne of explosives" probably means on

The original source was written by a journalist with (presumably) no military, science or engineering background, so "one tonne of explosives" probably means one tonne of artillery shells, most of which is steel. Still, 1.2 billion tons seems excessive, since that's the equivalent of China's entire production of steel in 2015, or 40 straight years of production for 1914 Germany.