I like the idea of an adjunct energy storage, and I can see why a salt tank, kept hot (like really hot. these things can be 500°C or more (even the cold tank is just below 300°C) and the "salt" is sodium or calcium nitrate type stuff) does the trick because heat exchanger back to working liquid for the turbine, thats well understood.

But I also would have thought a battery stack would perform exactly the same function as the salt tank: long duration storage of energy, available to supplement the nuclear power when required.

I believe there's some other reason this specific coupling of a reactor and a heat store makes sense which I didn't get from this article: Maybe it provides resiliency for thermal systems management overall?