As someone who's interested in all this, I agree it would be nice to have more precision around capacity. Especially as it relates to longer term storage. But! In this context, output is more salient than capacity. You'll see a lot of stories about grid-scale storage that use output. (https://physics.stackexchange.com/q/854999 offers a fuller explanation than what I'll give here.)

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!