From the flow graph, the film grain estimation is done on the difference between the de-noised video, and the original. So it only picks up the difference that

From the flow graph, the film grain estimation is done on the difference between the de-noised video, and the original. So it only picks up the difference that the denoiser creates. > and estimating the film grain parameters from flat regions of the difference between the noisy and de-noised versions of the video sequence.