As I understand it, the article is about the frequency stabilisation market. The existing reserves are able to pick up the loss of energy eventually, but reliability of the grid depends on more than just total reserve power. That is why the frequency stabilisation market exists. As the article points out, no one was expected to lose power here.

Power systems are highly nonlinear. At any given time there are power generators spun up and able to deliver more power. In the case of hydro, for example, vanes can control the flow of water. More water at one generator means everyone's turbines can spin slightly faster. Throughout the day, many such adjustments are made to match the load with the supply, e.g. with predictive adjustments every 15 minutes, say. But the systems can only respond so fast and each station can only pick up the frequency so fast. When a large enough generator drops out, or load suddenly increases (lots of people turning their kettles on during an ad break on telly, for example), numerous things can happen, including increasing supply, reducing loads and tripping breakers. In the worst case, some consumers lose supply. The advantage of electric batteries, being pointed out here, is that they can react in milliseconds, helping to stabilise the supply. This can prevent those corner cases where dropping consumers from the grid, due to cascading failures (highly nonlinear events), is the alternative.

The issue is that it helped stabilise the frequency which prevents circuit breakers kicking in and possibly taking South Australia off the air like it did previously. In other words it did exactly what it was bought to do.