Why would he not rate his own app 5 stars? Same reason every presidential candidate ever says they are going to win the election. It makes no sense to say otherwise.
I think most presidential candidates actually believe, that they are the best choice. As a programmer you know where you took shortcuts. I think I would not honestly rate any software I wrote to be perfect, because such software has not been made yet. On the App Store I would probably just lie and give it 5 stars anyway :)