I was using the camera to clip through the mask of the techeuns and when you clip through you can see they have eyes. However, when I did the same thing to the corsairs, I saw they lacked any eyes. Now maybe Bungie just didn't bother, but I thought it was weird that Bungie would give eyes to the techeuns but not corsairs. Perhaps corsairs canonically don't have eyes? They have those masks over their eyes after all. It's possible they have some kind of interface wired into them, making their eyes useless. As for the techeuns, because of their powers, they still may need their eyes. In the opening to The Taken King, when Mara and her techeuns are using their powers, her eyes begin to glow more than usual before she closes them. It's possible that this is the same for the techeuns, which would explain why they still have their eyes.