To create an app preview video, it is easiest if the developer records the screen of all type of devices, when using the app.
What if I have only one type of device, an iPhone 5? If I create app preview video and upload only for the iPhone 5, what will happen when someone with some other device, like iPhone 6, tries to see my app?
Will he able to see as replacement video, the video for iPhone 5, or no video will be in App Store to see because video is strictly device-specific?
Sorry for the very very very late reply.
The reason you have to upload different screenshots for different devices is so the user gets the most optimized screenshots. If you only have one device, try to record the simulator at 100% scale on each device so you can get the app preview on every device.