Patenting chalk water solution is like patenting milk.

Oh look, I've made up a liquid consisting of suspended lipids, sugars, and proteins! Please detain these cows!

These corporations would try to patent any molecular arrangement that contains two oxygen atoms and call it a day and they'd fight a plant for it.

Well spotted

More information is alway s useful. But it's pretty obviously quicker to build because it only needs to handle 40% of the liquid and it's not on a mountain.

Again, we can use the water for things, and water is something we can get more of one way or another.

A 2.5x multiplier doesn't seem as worth it to me, especially when we can do stuff like add hydrothermal storage to that number easily, among other things.

Entirely true, but since we're talking volume, this is only a 25% increase in linear dimensions (for the advertised 2x increase) or 35% (for the 2.5X maximum slurry density). If we are limited to a specific height of retention, that's 40% and 60% (rounded). Note: for structural capacity, like a tank, retaining a g=2.5 liquid requires substantially higher strength than a g=1 liquid (for a given retention height). Since this is the internet and should source my knowledge: I know this because I happen to be an engineer who designs retaining structures. Anyway...

For the effective cost of creating and maintaining the slurry, maintaining the integrity of the system (and keeping out wildlife), and the cost of decommissioning the otherwise unusable fluid, you're likely talking about a reduction in area of 20-38% (1/8) to switch from using plain water to this engineered material. I don't disagree that there may be some edge cases where the increased risk and expense is justifiable, but it's hard to see this being viable except as some kind of tech demo.

I actually bet it's just very very fine stonedust and water in suspension.

Pumped storage is great, but it is only a partial answer. You describe the "supply shaping" aspect of grid management, where we assume the demand for power is outside of our control, and we adjust our supply to meet whatever is being demanded. The problem is that pumped storage is not scalable. There are only so many places you can build reservoirs.

We need to focus on "demand shaping". Instead of (or rather, in addition to) putting power into storage so we can take it back out at a different time of the day, we need to shift our consumption of power so it is used when it can be produced.

Supply shaping is the flattening and shifting of the supply peak to match the demand peak. It is far more efficient and scalable to flatten and shift the demand peak to match supply.

We need large appliances like home and commercial water heaters and deep freezers to be directly aware of grid conditions and temporarily adjust their setpoints a few degrees.

Steel production, aluminum smelting, and other heavy industries are commonly done overnight during the demand trough, where they increase the base load and reduce reliance on peaker plants. They need to be shifted to daytime operation during the supply peak.

We need massive energy sinks such as desalination plants, hydrogen electrolysis plants, and Fischer-Tropsch synfuel production plants collocated with and powered by solar, wind, wave, and tidal energy facilities. They need to suck up any cheap, free, or "negative rate" power they can get, but shut down production and back feed the grid with their own generation when prices rise during the day.