Would make more sense if you reported what fraction of the entire night energy demand was offset by batteries. Which eye-balling it, still appears to be a healthy amount.
The early evening time frame is actually pretty important. Demand doesn't fall until later in the evening, but solar supply drops out.
Before large battery capacity, this demand had to be satisfied with (expensive) peaker plants and imports. Looking at the graph from that one day, there's still a big jump in supply from natural gas until overall demand falls, but imports come on gradually.
Of course, this graph is only from one day; but if this is normal and especially as more batteries go online, it may significantly reduce the use of gas peaker plants and should reduce the cost of early evening electricity. Although, if you still need those plants for grid stability in rare events, using them less regularly will likely increase cost to use them when needed.