This is likely it, though I don't know the particulars of this grid. In most grids, there's a small amount of super profitable frequency regulation to be performed. However, there's very limited capacity needed for that. Doing energy arbitrage over time is much more difficult to make profitable, usually. A huge amount of new solar and wind in the US will be built with batteries attached in the next five years, because it's easier to make energy arbitrage work when you're an energy producer (and also make the tax credits for storage work, which typically require that the batter is only charged from zero carbon electricity).