80 MWh is massive for a battery. MW are a measure of power, not energy. You mean MW-h. It's incorrect to say something has a MW capacity when discussing energy storage.
He's quoting TFA,
> With a capacity of 20 MW/80 MWh
It would seem sensible that the system has both metrics: a total capacity of storage, and a total rate at which it can supply that storage to things connected to it.
Further, my understanding is that the parent you're responding to is correct: pumped-storage hydro tends to have much greater generation rates; Wikipedia lists an example of 360MW.