Weather forecasting is actually really important for military operations. Consider weather advisories for aircraft, for example. Or planning an offensive on a clear day.
That said I don't know if this place was doing climate science or weather forecasting (or both).
If Apple chose to use a 16-bit integer for the index, they would be limited to approximately 65,000 photos. 32-bit would be about 4 billion. Photos likely uses a 32-bit or even 64-bit integer index value to make it a non-issue.
There could be issues with having too many thumbnails loaded in memory. Maybe some other value overflows when scrolling through (or just loading) an enormous library. If Photos also creates an index for searching that could also create headaches. If Photos were a simple file explorer having a million files shouldn't be an issue but it's more than that, so it's good to know it seems to be performant with even 1 million photos.
Weather forecasting is actually really important for military operations. Consider weather advisories for aircraft, for example. Or planning an offensive on a clear day.
That said I don't know if this place was doing climate science or weather forecasting (or both).