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Based on the video found on the Energy Vault web site they seem to be doing some serious engineering in building their prototype:

https://energyvault.com/commercial-demonstration-unit/

As to whether it works, I guess they will know once they've complete the prototype build and done some testing.

Just do the math. Total amount of energy they store is no more than a TEU container worth of lithium batteries, or few dozens of modern flywheels, or hundred so of plain steel ones.

Yeah all of them are questionable. The only one that has even a tiny chance of succeeding is the piston idea. You don't have to pay for the weight and you only have to cut the circumference of the piston out.

The piston idea has a problem. Rock isn't like that. Natural rock does not often come as one big solid piece. It's more like a pile of boulders of different sizes. There are cracks, underground water, rubble, etc.

Can you provide a source for this?

Here's a sarcastic YouTube video, at least: https://www.youtube.com/watch?v=NIhCuzxNvv0