I'm not really sure why you couldn't use this to uniquely identify any device, calibrated or not, that wasn't adding noise.
They're using the discrete nature of the sensor output to let them work out the device-specific calibration values (integer sensor output to calibrated real numbers). For uncalibrated devices those values will not be unique (I'd guess batch-specific at best?). If you could find a way for the uncalibrated devices to secretly determine their correct calibration then I suppose that might be identifiable, but calibrating something is difficult enough, let alone without co-operation, and the calibration uncertainty is likely to be more than the spread of device characteristics anyway. Attacking calibrated devices you're really reading a set of stored floating point numbers that are the same every time you extract them.