My guess is that the pediatrician has had too many parents who find that their child is interested early, but then becomes uninterested again in a few weeks or months, reverting back. This isn't unusual. The problem is that many of the parents of these children will say to themselves, "If she could do it then, she can do it now!" and force the issue.
I would say a better approach would have been, "Sure! Let her try it out now, because it is a game for her. But don't be surprised if she, in a few months, isn't interested and starts to have 'accidents' again. Don't worry. Don't force the issue. Put her back in diapers until later."