an MFDB is mirrorless. I can put my back on Hasselblad V or Hasselblad Flexbody. But I think that LV and EVF are missing components. Latest backs have some LV capability.
I think he meant something like a Mamiya 7, a fully integrated camera system, backs are mirrorless but also camera-less too, and if you put one on a camera they bulge out. You pay twice in size/weight for having electronics both in the camera and back, instead of a unified system.
I just had a thought - if Leica made the S system "built from the ground up" as a purely digital camera system, why did they have to make it an SLR? It's a concept that's analog in nature (getting around the capture medium), and it only seems they did that because they went with a CCD sensor. The 30x45mm size they picked isn't all that big, if they had ordered a custom-size CMOS chip, they could've had a fully electronic medium format camera by now.
It seems some people hate progress with a passion.