It is using computer vision to compare the instagram photo with the recorded 'open camera' footage to give the best guess at a timestamp. You can see the CV drawing boxes around the objects it is identifying in the footage on the right side, and it already knows that the photo was taken at the location (since the location tag was used that to scrape the photos), so it is basically just hunting for a person that best matches the subject in the instagram photo.
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It is using computer vision to compare the instagram photo with the recorded 'open camera' footage to give the best guess at a timestamp. You can see the CV drawing boxes around the objects it is identifying in the footage on the right side, and it already knows that the photo was taken at the location (since the location tag was used that to scrape the photos), so it is basically just hunting for a person that best matches the subject in the instagram photo.