This way you don't need to design your app to work with different aspect ratios, because it's always the same.
But you still do need to support different aspect ratios if you want your app to work on any other iPhone models, because many of them differ.
Even if it were true you would also have to design the app for when the device is rotated.
But you still do need to support different aspect ratios if you want your app to work on any other iPhone models, because many of them differ.