Isn't the focus screen there just to allow short back focus lenses like the EOS-M mount to actually focus with the standard DSLR flange depth? The mirror flips down to display the through-the-lens image via the ovf and the AF focuses the lens. When you press the shutter, the mirror flips out of the way and (in this case) the sensor flips up to the line of sight and exposes the shot. If you're shooting in mirrorless mode, the mirror moves out of the way and the ovf stops functioning and the sensor flips up into place and initiates liveview focusing/shooting.
It does seem like a very complicated setup where you lose the mirrorless benefit of no mirror box therefore a slimmer body in order to gain the choice of both options. I guess there's the risk of it being a jack of all trades but master of none.