I also already pointed to this plain surface under 'the other half' but I don't think that Jolla is lying about the (boundless) possibilities of the backcover.

I think that this surface under 'the other half' is an over removable cover under which you can see the battary and card slot and other things like some contacts to the attached hardware... They never showed us the back (without the other half) exactly till now and nobody came up with the idea of asking to show the battery.
So I think you can also replace this black cover with an other one which is adjusted to specific 'other halfs'' with e.g. cavities for some contacts...

Edit:
BTW: I just reading this and don't believe anymore that Jolla considered the option of a hardware keyboard because SailfishOS is only working in portrait mode at the moment...