In northern Switzerland, a construction team is hard at work excavating a hole in the ground that will end up being over 88 ft (27 m) deep, and spanning the length of two soccer pitches. This pit will be home to Switzerland's first redox flow battery for storing clean energy – and it'll be the most…
From what I read, it does 800 MW max, which is still a lot.
1.2 GWh is just the capacity. And response time can indeed be quite fast as the system is always on standby, but I would guess it would still take a few seconds to go from zero to full power
But with 800 MW it takes an hour and a half to get to 1.2 GWh, and an hour is 3600 seconds or 3600000 ms. So “a few” is doing a lot of work in that quote.
The quote is obviously incorrect, perhaps the engineer meant to say “absorb 1.2 GWh and start providing power to the grid in a matter of milliseconds”, and it was lost in translation.