The nice thing in this process of the oxidation of the metal, it is done with hydrogen and does not involve any emission of CO2. Effectively what they do is:

Electricity from renewables -> Hydrogen (electrolysis) -> (iron oxide to iron, in loop) -> heat -> iron oxide.

I would say this is a nice solution to the problem of storing hydrogen.

EDIT: Source, the publication: https://www.sciencedirect.com/science/article/pii/S036012851...