Most grids have a rapid response system like this. They’ll have contracts with very large power consumers (think big hydro pumps, metal smelters etc) that can afford to have their equipment power down without notice in exchange for energy rebates.

Then when the frequency drops these consumers will automatically turn off their equipment to reduce power draw.

The seeing such a huge drop like this is extremely unusual, no doubt many systems immediately started load shedding (both voluntary and involuntary) to compensate