There are three ways you build up the expertise to go FF mirrorless:
1) Start with a smaller sensor and experiment on EVF, AF systems, handling, etc. Iterate and improve. Once you have a slick, compelling product, you eventually get brave enough to step up to FF.
2) Defer the billion dollar mount decision and offer a fixed lens FF rig. Do the same -- experiment, learn, iterate, improve.
3) Pull the mirror out of an EF rig and let them use EF lenses out of the gate*. It may not be the final product you want to use (will be far thicker than needed for the 'please please please keep it smaller than my SLR' mirrorless faction), but this rig would have zero risk of dropping the ball on grip, controls, ergonomics, etc.
* yes, of course, 'The I don't need it to be smaller' faction wants this rig as the go-forward FF mirrorless system as it will seamlessly use existing lenses without adaptors.
All that said, I would have thought #3 would have been the play before #2 ever happened. But, if this story is true, Canon seems to be taking a page out of Sony's book, as they of course did the same thing.
- A