The email is confusingly worded, but I think it's simply saying that 1. you can't require account registration if the app does not provide account-specific fun

The email is confusingly worded, but I think it's simply saying that 1. you can't require account registration if the app does not provide account-specific functionality, and 2. if your app has both account-specific and non-account-specific functionality, registration can only be required to access the former. I'm going to agree with houseabsolute and chalk this up to a mistake. That is, the app reviewer mistakenly assumed the app did not have account-specific functionality, or exclusively account-specific functionality. There are far too many (major) apps that require an account to do anything for his interpretation to make sense. I wouldn't put this sort of craziness past Apple, but reviewer error just seems far more likely.