"But in reality, the response from the Tesla big battery was even quicker than that – in milliseconds – but too fast for the AEMO data to record.
Importantly, by the time that the contracted Gladstone coal unit had gotten out of bed and put its socks on so it can inject more into the grid – it is paid to respond in six seconds – the fall in frequency had already been arrested and was being reversed."
TL;DR The reserve system did this to show its skills. It can respond faster than any existing base load generators, and can derive revenue not only from providing energy on demand, but also grid services (in this case, frequency response).
Tesla plans to provide these services in a virtual fashion with vehicles in the future: https://i.imgur.com/oLBNeW4.png