They compare its cost effectiveness to lithium electrochemical storage batteries, but it seems much more apt to compare it to large-scale flow batteries, which also use relatively cheap, easily available materials. How does it compare to those?
(I do believe the article's design fares much worse, even on capex alone. I never saw some salt selection that melts at 600°C, but I imagine it would have better results even in a lower temperature.)
Iron flow is currently 'it will be dirt cheap later we swear, ask us about a demo project'. So probably substantially more expensive. No compelling reason not to believe them though.