Originally Posted by SurfHenk

As a backer, my understanding based on the information at hand is that the seat post is round.

And indeed, the seat can slide fore and after in order to achieve a compact fold when needed and yet allow to adapt the fore/after position of the saddle optimally to the rider.
The information provided to us backers is that there is a way to (slightly) alter the tilt of the saddle up or down by means of a dedicated screw.
And yes, the seatpost should accept any standard-railed saddle as long as it does not interfere with the fore/after mechanism and lever. I have no idea (yet) how restricting this might be wrt saddle choice..