The 500% is quite reasonable. The hydrogen part isn't.

85% of hydrogen is manufactured at site of use for multiple reasons, but a big one is that it is a complete bugger to store and transport. No one is shipping hydrogen internationally today and there's no reason to think anyone will tomorrow.

What will happen is HVDC cables will take the excess electricity away from SA's grid to the rest of Australia and to the rest of the world. There's already a massive solar farm going in whose electricity will be shipped to Singapore via a 4,600 km underwater HVDC cable. That's the future of energy distribution, not pressurized tanker ships of hydrogen.