That's not true totally, since you can side load on Android it's trivial to spoof an App Identity to match what the facebook app is expecting. You can get the Identity from the original app. What is somewhat more difficult is doing this across a bunch of phones since the Play store won't register your app if it's spoofing another apps id. In the end it's just a series of hoops to jump through like almost all security. The more hoops you put between you and the bad guys the less it's worth jumping through them, in theory that is. There will always be the guy who has to know.

*edit I mean it's not true that the attacker can't impersonate your app. Your description of how it works looks right on to me. :)