None of the Zero Image pinhole cameras are rectilinear by design. Point them up or down and distortion of lines and verticals will be very noticeable. A few ZI cameras have the troublesome zone focusing in place instead of pinhole (a couple more still have both options to play with!); ZF can result in very indistinct images as opposed to the better clarity of pinhole. ZI pinholes are true pinhole cameras — far removed from the HOLGA, which is a lens camera.
Film is held very flat in those ZI cameras with removeable/slip-in backs which press the film against the format box. I would be more concerned with the film curl at the scanning stage.