I think he used something like z = c * dy since earlier in the post he mentioned the photos were from a similar direction but different elevations. He compared this to your how your eyes compute distance but in this case the difference is vertical.
And don't forget "In this case because the precise location and elevation of the photographers isn't known this is slightly more art than science, but it is still fun!"
And don't forget "In this case because the precise location and elevation of the photographers isn't known this is slightly more art than science, but it is still fun!"