I would also assume this is the case. If you bear in mind that in real stitching the thread goes up and down through the whole wing, or up and down through a strip of fabric glued to the rib, then it would be difficult as well as pointless to "sew" the stitching over a solid surface. And, of course, the purpose of stitching is to prevent the fabric from "ballooning" due to pressure differentials. So again, no reason to have stitching over sheeted sections of the wing.

Does anyone happen to have a photo that shows a wing with stitching that only extends for part of the cord?