I hadn't taken into consideration that some hydro generation may...

I hadn't taken into consideration that some hydro generation may be in conjunction with environmental water releases. I had assumed that at this time of the year it would not be water releases because the dams were at capacity, and I was also assuming that the water being pumped up would not be into the same reservoir being used for generation.
The big advantages of hydro is the huge amount of potential energy in the form of water that can be held in the storage dams, and the relative small cost of accumulating it through the collection of natural runoff.
I can see how pumped hydro could be useful if the situation ever arises that electricity produced by renewables is in excess of what can be utilised by the grid and has to be dumped somewhere, but that is nowhere yet near being the case.
What is happening now from the perspective of the whole grid, is that pumped hydro is being effectively powered by non-renewable sources such coal and gas. The electricity used pumping the water back into storage if instead was just fed into the grid would reduce the usage of finite sources of energy and allow more coal or gas or water to be retained in their respective storage's for usage when the intermittent renewables are not capable of meeting the demand.

The same applies for the battery. The claim that it can supply an almost instantaneous injection into the grid faster than any other sources was put to rest recently when the Loy Yang unit dropped out and it was Gladstone that responded not only quicker, but was able to go one and supply the shortfall until the no longer required.

I do agree however with one thing, that being that it's all about being taking advantage of opportunities in the market to buy low and sell high which rather than increasing competition and reducing real prices for the consumer actually represents the reason power prices have become as high as they have.