Worth noting that because he's using a hydraulic press, the load isn't constant. It deforms, fibers break, and thus the load decreases as the deflection stays

Worth noting that because he's using a hydraulic press, the load isn't constant. It deforms, fibers break, and thus the load decreases as the deflection stays relatively constant in that time frame. With external pressure the load doesn't decrease. Fibers break -> deflection increases -> more fibers break. There isn't load relief as long as the vessel is still holding pressure.