Supports iPhone only when sent to Apple

I submitted an update for my app to iTunes Connect that Apple declined, giving the following reason:

We noticed that your app was not launching at iPhone resolution when viewed on an iPad running iOS 10.3. In particular, the screen resolution has not been optimized for the iPad.

But I have installed the iPhone support only app in Info Info, under Devices, so I don't understand why they would be testing the iPad.

I tried to figure out how to make sure the iPad is not supported, but I don't have a storyboard for iPad and I only uploaded iPhone screenshots to the app store. How can I fix this error?