Feature fragmentation (Apple's way) means that Bob's new 4s gets 100% of the updated iOS 7 base apps, 78% of the major new features of iOS 7, and the ability to run third-party apps designed for iOS 7.
OS fragmentation (Android's way) would mean that Bob's new 4s wouldn't get an upgrade to iOS 7. Bob's new iPhone 4s would be stuck, forever, on iOS 6.
To your point, yes -- under Apple's method -- Bob won't get AirDrop. Oh, wait indeed!
Using Google's method, not only would Bob not get AirDrop, he'd also not get Control Center, Filters (in Photos), the new camera app, the new multitasking, the new Notification Center, iTunes Radio, or the updated base apps.
If you're Bob, please tell me why he'd like Google's method better than Apple's method?
Plus (with Google's way), Bob would be stuck with iOS 6 versions of all of his third-party apps. Bob won't be alone in this situation, which now creates a situation where 90%+ of all iOS devices aren't running the latest OS version. Uh oh, significant OS fragmentation. So now developers have to not only develop and support iOS 7 versions of their apps, now there's a much larger iOS 6 audience still out there, so developers have to spend more time supporting and maintaining that version too.
There may be a better demonstration of feature fragmentation being worse than OS fragmentation. But saying "Uh oh, Bob's new iPhone 4s can't do AirDrop" doesn't seem to be a good example, when the alternative is "Bob's new iPhone 4s can't do ANY FEATURE of iOS 7".