Currently XCode will build for iOS 7.1 and up.
However, it was built with the 8 sdk,
If it was built with the iOS 8 SDK, then it was an iOS 8 app (even if the Deployment Target was as low as 6.1). The fact that it happened to run on a 7.1 device is just due to the Deployment Target settings and your extra testing. With care you can even get an app to run on an iOS 5.1.1 device and get approved. But new apps built with the 7.1 SDK or lower are no longer accepted. Only iOS 8 apps.