My understanding is: 1) More megapixels, 2) a larger sensor (more than twice as large) means more signal-to-noise at each pixel site which means less noise in low light, 3) the Mamiya has far greater dynamic range (around one stop greater, double the amount of light).
You're correct. What I'm wondering is why they used a 1DX instead of a Sony A7S? The ISO range on the Sony would of probably outdone both Canon and the Mamiya.
Taking photos at ISO 409,600 would be pretty awesome IMO.
You don't really want photos at ISO 409600 per se, though, you just want photos at the lowest ISO that can get you the necessary depth-of-field for your shot (here extended with tilt-shift lens) at whatever shutter speed avoids being too shaky. And then you want as high of a dynamic range of possible, so you can distinguish the brightest brights from the darkest darks in a single shot.
If you can do that already with the sensitivity you've brought, higher ISO doesn't buy you anything...
Why use the Canon 1DX? The photographer has a long-standing and public relationship with Canon.
Why not use the Sony A7S? The photographer uses a Canon 200-400mm f/4 lens, and the nearest Sony equivalent is 70-400mm f/4-5.6, which means at the long end it captures only half as much light, instantly negating any possible ISO range advantage offered by the A7S.
