This article doesn't really say, but gas power plants are frequently used for peak demand because you can easily throttle them to meet fluctuating demand (and generated power that is not used is wasted). So, even though we're comparing energy production (gas) with energy storage (batteries) it does make some sense because the batteries can probably replace the need for peaking plants eventually. If we can smooth out the demand cycle with storage for power generated during off-peak hours it would make the power grid much easier to manage.