I would think the energy density varies with that of the fuel they put in. They mention hydrogen, natural gas, gasoline, ammonia, butane, propane, alcohols, syngas…. That’s about anything that is or can easily be turned into a gas that burns.

also, “/liter”, for gases such as hydrogen, can be made larger by using higher pressures in your tank.

On the other hand, they also say “target efficiency: ≥ 40% wire to wire”, and 40% of 1250 is 500, so it may be that.