CMOS image sensors have lots of per pixel variation due to errors in the manufacturing processes. Examples are the "dark current", which causes patterns sometimes in images. Thats corrected for in software by all modern cameras.

99.5% accurate suggests successfully isolating 8 bits of device-specific data from a couple of megabytes of image data.

It is no surprise whatsoever that the basic corrections done on the images leave behind at least 8 bits of entropy.