It looks like the reason for the custom timestamp setup is to assert and upper and lower bound on time. A normal timestamp server can asset it saw the image at a certain time but not that the image wasn’t created much earlier. This setup, the image processing pipeline can immediately attach the last seen timestamp to the photo as a lower bound, and then connect to the network to get the upper bound time.

If there is too much of a gap between the upper and lower bounds then the image becomes suspicious.