This is a lens adaptor, not a mirror box. Its needed to be able to adapt A lenses to a E mount body, and still get autofocus and exposure control.
Canon makes a EF to M adaptor, but you don't need a mirror in order to get AF and Exposure control because it was planned on in advance.
Allow me to explain my thinking further. The existing AF in the M is only fair (not that any of Canon's cameras are great when using Live View). If Canon crafted an accessory like this where, using a pellicle mirror, they could have a much more sophisticated AF that wasn't sensor based, then it might be an interesting option for those times when you want a really top notch AF.
The suggestion "get a DSLR" misses the point, because there are plenty of times when a DSLR is not appropriate. While 'get a DSLR and an M' might please Canon's accountants, I was thinking more of the opportunity to capture people upgrading from a P+S, by allowing them a lower cost upgrade path (without buying a new body).
And the referenced adapter clearly IS a mirror box, as it contains a mirror specifically to enable Sony to bring a separate autofocus and exposure sensor into the game on a mirrorless body.