If it works on children who have never been able to hear, then it should work on adults who have lost their hearing.........
It sounds like a genetic fix, which is probably a gross oversimplification.
Adults losing their hearing is probably physical damage to the mechanism of the ears.
The included image is cute, but I doubt that the cells of an afflicted person get their DNA repaired and start producing healthy cells. I won’t be surprised at all if the hearing improvement isn’t lost over time, sadly.