This. McCoy even did it when he gave the old woman in TVH a pill that grew a new kidney (very fast). That technology must've been so common that he had the pill "in stock" in his little black bag.
Let's say Khan has restorative nanobots in his blood. That is an extremely credible science fiction concept compared to the "big three" of Trek science that hold the franchise together: warp speed, transporters, and artificial gravity. Those three are anywhere from infeasible to outright impossible. I'll even believe in "magic blood" before I believe a universal translator and that there are so many humanoid aliens with funny foreheads out there.