Would not last long, waste of time.

In nederland wordt ie onder Flamco verkocht maar staat ook nog op vernoemd powerd by Sunamp

See midsummer Cambridge

Solar power or off peak energy… it’s designed to lower the costs but for how expensive they are it would need to last a long time to save you anything

The main selling point is a compact size. The energy source is irrelevant. You could use water if you don't care about space. This is a heat battery, not some miraculous energy efficient heater. Yes, you could go with an instant water heater instead which takes even less space, but there's no storage. Why would you need to store heat? Well, the problem is that energy availability is uneven throughout the day. At peak hours it costs more as it needs to be either imported or running an additional less efficient power plant. With solar it's obvious: sun doesn't shine at night. And wind turbines don't work without wind. And as you mentioned, heat pumps, they're kind of slow, need more time to pump heat to where you need it. So batteries are kind of a big deal. LiFePo batteries can store electricity, but they're quite expensive. Water can store heat but it takes space and has a higher heat loss. The salt is comparatively cheap and and due to state changing properties has a high energy density and slower heat loss.