For batteries you need two measures at minimum: power (what you can get out when the battery is fully?/half?/?? charged) and energy. But there's more because a battery's power output might not be stable, and it might not be linear with remaining charge.
Don't stop reading there! The next three words are "on the nation's grids" (at least in the first occurrence of the phrase). When dealing with grids, capacity always refers to power measurements.
The journalists did not mess up anything, they are actually using the correct units used by people who run grids, build grid assets, plan grids, etc. The only people who are confused are those who don't know about grids, but know a few things about electricity in other contexts.
Grid scale batteries are treated as alternate supplies of energy, but yes, they are storage, and cannot indefinitely run at a constant output.
That's the confusion. Directly comparing them is not useful, except when describing a finite time span.
Gigajoules doesn't really help clarify the confusion, because it doesn't change that they're not directly comparable.