can anyone speak to the non-obvious (like more megapixel) advantages of shooting this on the medium format Mamiya that he mentions? I've always been intrigued by MF cameras, but they cost so much more causing them to be inaccessible (to me anyway).
It's an overall larger system. A larger sensor means less noise (assuming fewer pixels per inch, which is true in this case). A larger lens means less distortion (because any imperfections are smaller relative to the entire glass).
And it's very expensive, which means even a relatively small advantage in image quality will make the resulting pictures more valuable, at least for a few years until someone else does this again with newer technology. DSLR photographs are truly commoditized now, but medium format are not, never have been, and probably never will be.