The article says they will use a byproduct from a local industry, perhaps it's available for cheaper.
"The sand itself will also be sustainably sourced – it’ll consist of crushed soapstone, which is a manufacturing byproduct of another local industry. This material can apparently conduct heat even better than regular old sand.".
Cheaper than water?!
Hot sand / crushed rock doesn't have the same problem. If you read in the linked article it says
> with the sand heated to somewhere around 500-600 degrees Celsius (932-1112 °F).
That would be extraordinarily hard to do with water as you'd need significant containment and safety measures.
Also, water tends to make for a horrible heat battery as it is much more thermal-emissivity than rock particles. You need all sorts of additional insulation to retain the heat, whereas the sand will insulate itself.