1. Decompressing the gas can be used to do work, like turning a turbine or something. It's not particularly efficient, as you mention, but it can store some energy for a while. Also the tech to do this is practically off-the-shelf right now, and doesn't rely on a ton of R&D to ramp up. Well, maybe the large storage tanks do, but that should be all. So it _does_ function and nobody else is doing it this way so perhaps all that's seen as a competitive edge of sorts.
2. The storage tech has viable side-products, so the bottom-line could be diversified as to not be completely reliant on electricity generation. The compressed gas itself can be sold. Processed a little further, it can be sold as dry ice. Or maybe the facility can be dual-purposed for refrigeration of goods.
3. IMO, they're using CO2 as a working fluid is an attempt to sound carbon-sequestration-adjacent. Basically, doubling-down on environmentally-sound keywords to attract investment. Yes, I'm saying they're greenwashing what should otherwise be a sand battery or something else that moves _heat_ around more efficiently.