In this context it certainly refers to the propellant. There is no way you can fit 230 kg of explosive in a 500 kg projectile. For reference, the US Navy's 12" (305 mm) HC shell from WW2 had a 36 kg bursting charge in a 426 kg projectile. You'd have even less volume for explosives if the projectile is saboted. I can see this 300 mm artillery piece being useful for long-range strikes against high-value (including underground) targets, but for saturated artillery bombardments on the front line, you really need the volume of fire that only 122 and 155 mm artillery can deliver.