I'm confused - from what I understand, this "Birkin Pochette" is made to sit on the inside of the bag, but against the inside of the front, not the back. The handle openings hang around the front handle. I suppose it could sit, unhooked, against the back of the bag, against the tucked-in flap.
I also believe that it was shown at an angle on the runway to illustrate that it is not affixed to the bag. It appears to me that the Birkin itself is a regular Birkin, and that it just comes with this Pochette that can be used with the bag or separately.
I am wondering whether the Birkin Pochette will also be sold separately, but I'm sure it would be, ah, challenging (to say the least) to find a match for any particular color bag.