It looks like their current generation of systems are electricity in, via resistive heating, then heat out via hot air. Efficient electricity out would appear to be an active research area for them, but it would also seem, that there are still a number of applications where on demand heat at several hundreds of degrees is still very valuable.

There is a sand battery start up in Tampere (Finland) that is heating to 1k Celsius so by the OPs calculations that would still give it a small edge over water but with the added bonus of the intrinsic insulation - very handy in the cold winters. They have a pilot plant that is being developed to support the municipal heating network.