They use very large blur radius. At this radius rounding to 8bit + lossy image compression should be destructive enough. It's impossible to tell from the scree

They use very large blur radius. At this radius rounding to 8bit + lossy image compression should be destructive enough. It's impossible to tell from the screenshot, but if they're smart, they should have an explicit degradation step before blurring (e.g. pixelate/lower resolution first).