Yes, and in my example I showed how devices preloaded with the "latest current" version of iOS, which at the time was iOS 4, were unable to get some of the major features of the "next version", iOS 5.
Apple gives the same name/version to two different operating systems. One includes Siri, one does not.
Would it have made you happier if Microsoft said "Everyone gets WP8! Except without true multitasking and new app support." Because that's exactly what Apple did when it said the first two iPhones wouldn't get multitasking.
But I can see the confusion... Apple hasn't announced any iOS 6 features that are incompatible with the iPhone 4S. That's just because they haven't announced the iPhone 5. If they stay true-to-form, they will introduce the new iPhone in September and it will include an iOS 6 with more features that are not compatible with the iPhone 4S. Just wait and see.