IIRC, when the 5DM2 came out, video was a new addition to DSLR bodies that had for decades been designed for "stills only". The Set button to start recording? Clearly primitive controls, but we didn't care, because we had a FF sensor, 1080 video that rocked in low light, and existing inventory of lenses!
When the 7D came out, there was more time to redesign the buttons to make dedicated video functions and tweak the layout, (mostly) all welcomed.
So looking at a 2012 prototype, seeing a button placement, and then concluding that the "inner guts" of the camera must be/must not be either a 5DM3 or a 7D2 seems rather spurious, at best.
Wouldn't Canon take all their collective "best ideas" (past and present, at least in terms of button placement) and put them in to play in a new prototype?
I'm just thinking that us amateur detectives should not "read too much into" the potential inner workings of the camera based on button placement on prototype vs button placement on "rushed to video" 5DM2 or 7D.