No, you're actually reading it wrong. Its a unit of power, not energy. You can follow the link on that line and see the EIA also using watts.

It's talking about the ability to supply power to the grid, not how long it can supply that power. It's arguable how useful a measure that is but they're certainly using it correctly. IMO they're also talking about it in a reasonable way- capacity and power are pretty closely related for batteries, and the ability to pick up demand is critical to handling instability.