It's worth pointing out that in this case, enzyme replacement therapy was used in utero to treat the disease, not cure it. The therapy hasn't changed that the patient still isn't able to manufacture the needed enzyme, and barring new developments will need to continue the infusions for life. What's important is that usually the therapy is not begun before the child is born, but that means that glycogen buildup has already had a chance to affect fetal development; in this case (some of) that damage was hopefully forestalled by beginning treatment before birth. The article also mentions that the hope is that the body's allergic reaction to the enzyme infusion will be ammeliorated by beginning the treatment early, which strikes me as reasonable enough but it looks like they'd like to start collecting data on that to see if it's a justified benefit.