Yesterday I\'ve received a mail from the Apple Developer Center, stating:
Make sure your apps work seamlessly with the innovative technologies in iOS 7. Starting
I don't believe this is the case.
As long as the app is compiled using XCode, and using the iOS7 SDK, you should be fine.
When they say "built" they mean, it has a corresponding XCode project. Not that every line of code has been written in XCode. At least, that's my interpretation.