They’re using the stored heat energy as heat, not converting it back to electricity.

And if it’s very cheap, does it matter if the conversion is wasteful?

I'm not saying it's bad, but it's clearly not a replacement for batteries, given its applicability is quite limited.

The question is about conversion is, is it still cheap if you add a powerplant (i.e. converting heat into electricity) and have to maintain it (moving parts, in contrast to batteries).