Maybe that size includes the redundancy with all tiling layers of different resolution?
edit: reading the link in the GP closer, they seem to refer to the full imagine, but it looks like they extracted the drawing underneath the painting. So I'd guess something must be from outside the visible spectrum.
In this video [1] he mentions that each 100mp image is around 600MB, which aligns with the size of the 4-shot multi-shot mode of the H6D-400C MS. 8439 photos 579MB would be around 4.7TB in raw format.
