They're not dealing with a pressure differential. Or at least I don't think so.

I don't think the Journalist who wrote the article understood the technical details, but from digging a little at their website I think what's going on is they're moving heavy brine up and down, all of it equalized with local pressure.

Despite them describing it as pumped hydro, I think its better framed as a cousin of the "chunk of concrete suspended over a mine shaft" style gravity battery. Replace the mineshaft with water and the concrete with salt.

Oh, right thanks for clarification. They are indeed not pumping just any salt water, but much heavier brine (which they get who knows where).

So if there is any leak in the system, it will kill local wildlife right, like the brine pools under ice in Antarctica.