You get out the energy by combining the iron and the oxygen but if you want your iron back you've got to supply it with just as much energy (electrical) and then some.
If you have a surplus of energy from renewables, it's a method of storing that energy in a stable medium than could even be transported in bulk to where it's required.
We are currently dumping wind energy when demand is low and generation high. Wind power is very cheap, but it costs the national grid enormous sums to force generators to stop producing - we should explore all possible storage methods if we are serious about reducing carbon emissions.
They seem to think the process is 80% efficient, which seems a reasonable start, and the oxidation of metals produces zero carbon.