1/5th or so of California power is from Hydro, and that could increase if the water supply was increased.
Using daytime solar excess to pump water uphill lets you get double duty out of the hydro and looks to be about the right amount of after hours energy use.
Sure, but hydro is pretty unique in that the hydro dams, generators, and grid connections already exist and it can store well enough to not recharge just for hours/days/weeks, but even months.
Isn’t existing hydro already utilized? I don’t get how that helps us scale it up to replace fossil fuels?
Not really, it's typically water limited and not run full out. If water was pumped up during sunny (or windy) days when there's a surplus you could reuse the same water over and over again.