The factual dispute here seems to be a matter of how do you count people who were in the trees? By witness reports, there were people in amongst the trees to the side of the reflecting pool mall. This makes sense as some forested regions are nearer the Lincoln memorial than the available open area at the far end of that pool.
The photos won’t show how many people were in the trees. Witnesses were never in any position to count them. We really won’t know.
My own guess, from aerial photographs, is that there was room for another 30K people in positions amongst the trees that were close enough to the Memorial that sitting or standing there would have made sense.
This suggests that there would in reality have been somewhere between 5K and 15K people who don’t show in the photographs but were in attendance.