Energy density is measured in Wh/kg.
Proclaiming an energy-storage density of 596 Ah kg−1 is meaningless, because the voltage is missing.
I could claim that our drive battery has an energy density of 10000 Ah/kg simply by lowering the output voltage sufficiently (using a DC/DC converter).
Also, seeing that this is a fuel-cell, not a battery: What does the mass data relate to?
Mass of fuel cell alone? -Then they should compare to other fuel cells, not to Li-Ion.
Mass of fuel cell and fuel? -Then it is easy to fake good results by combining an oversize large fuel tank with a minuscule fuel cell. .. Which, when taken to the extreme, equates to the energy density of the fuel alone.
-Which is also quite misleading. We fly some 500 km on 5 kg of hydrogen. Sounds nice?
Wait until we have added the weight of the fuel cells, the peripheral systems and the pressure vessels.
El Reg is not at fault here though. Whoever wrote that press release either was being willfully misleading or ignorant of the matter at hand.