This is really old technology. Some prototype vehicles have already been made (and are in use) with molten battery tech. Which begs the question: why isn't it more prolific?
I can't imagine a storage unit like this being energetically viable on any scale beyond demonstrating a functioning prototype. I don't doubt it would work in theory, but to just keep something the size/scale of the mock-up for the prototype at 400-700 degrees Celsius would take a lot of energy/power, and I can't see the battery sustaining its temperature without a significant amount of draw which would still offset the capabilities of conventional storage media. Also, they discuss how this is much more scalable than conventional batteries--again, from a design standpoint I don't doubt that, but from a functioning standpoint, the amount of energy it would take to sustain this would increase exponentially with an increase in size (volume).
Also, this is sufficiently vague:
The target is to make a grid storage device able to deliver a few hours of power to fit into a shipping container, while a home battery would be about the size of a basement freezer, company executives said.
Since a basement freezer really isn't that much bigger than a "shipping container" (and depending on the size of a shipping container, the freezer could be much smaller), I don't see the relevance of this statement.
Unless they're farther along than they let on, or MS knows something about the tech that is not conveyed in the article, I am surprised that MS dished out money on this, since it's not a new battery technology, and prototype units have been demonstrated a long time ago, with nothing in the way of industrialization to show for it.