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.