There is no piston. Where do you see one? Did you even watch the video? The whole launch sequence is shown there. There's a pressurized hot hydrogen tank that e

There is no piston. Where do you see one? Did you even watch the video? The whole launch sequence is shown there. There's a pressurized hot hydrogen tank that empties into the barrel. There's no "piston [...] driven up to speed, pumping the hydrogen". The hydrogen flows into the barrel on its own because it has high static pressure at the moment of firing. The presenter literally explains it in detail.