This is because grid operators are most concerned with immediate power output. They need to keep the grid balanced, and if they need a gigawatt to do it, it doesn't matter if the batteries have 100 GWh if they can only discharge at 1 MW.
Since the batteries described here are used primarily to handle the peak of the duck curve (https://en.wikipedia.org/wiki/Duck_curve) it seems like 4 hours of capacity (the article mentions that the lithium-ion batteries have 4-6 hours of capacity) is sufficient to get over that difficult hump.
Anyway, to get back to your question of how many GWh, if we assume that the batteries have 4 hours of storage, then we're looking at around 4h * 15.7 GW = 63 GWh of battery capacity. (4 hours is what I've seen as standard for lithium-ion, conservative if the article's claim of "four to six hours" is true.)
Hope this helps ease the peeve!
Some storage project list power output figures that can only be sustained for 90 minutes: https://en.wikipedia.org/wiki/Victorian_Big_Battery
"Battery energy storage is not without challenges, however. Lithium-ion batteries — the most common type used for energy storage — typically have about four to six hours of capacity. It’s enough to support the grid during peak hours as the sun sets, but can still leave some gaps to be filled by natural gas."
Power plants don't have capacity, really.
So you hear about the output GW, not the capacity GWh.
For battery plants, they might mention duration (4 - 6 hours), which makes converting to capacity in terms of GWh trivial.
So rated power will give you that for about 3 to 4 hours.