Except that they used to be, and even if their initial move under included some pretty stupendous gengineering, they are clearly still compatible with a surface environment and, as you say, still crave light based on the response of the ones who do sneak topside. If they'd genuinely succeeded in revising /all the bodily functions/ that are light dependent or light cued... they really shouldn't be either of those. We don't see health issues, no, and that's part of what makes my teeth itch so bad. If they're a different enough species to /not need light/, it /should/ have an impact on their compatibility with the surface! Just, ugh. The handwavium is way too handwavy for me to be happy.