The caveat is you can't put this energy back on the grid. It's been converted to heat and there's no conversion back. It's mostly useful for heating homes and water at night.
But thats the problem they are looking to solve… there are quite a couple of cold months in Finland and I think a large portion of their heating is electric/distict anyway so they might as well store heat rather than energy. This probably serves as a rather good balancing mechanism to the grid actually, and I could definitely see this be a thing I northern Sweden for instance , where electricity is basically free during the night and when it is windy it’s actually kind of a problem to get rid of it all.
If it’s indeed 600 degrees C, it can boil water and power a steam turbine. Back to electricity.