If I remember correctly (and it's been a long time since I read it), in the A.C. Clarke book, the cable was made after scientists were able to construct a strand of carbon atoms - in effect, a thread of diamond.
Just because we don't have the technology to do this yet, doesn't mean that we shouldn't think about it and even plan how we'd go about it if the technology was available. After all, that sort of thinking is the basis for the very best Sci-Fi.
"Goodness is about what you do. Not what you pray to."