I think that iPhoneOS.sdk is a symlink, so the real version that it points to can dynamically change if you update Xcode or whatever. I assume that they're suggesting that you pick an exact version to be sure that your builds will be consistent. It probably doesn't matter all that much for most developers, but I'd say that you should follow the advice to be safe.
I'm pretty sure that a newer iOS SDK can still build for older iOS versions. Similar to how a newer AIR SDK can still target older AIR versions. AIR knows how to tell the iOS SDK which versions it needs.
By the way, it's worth mentioning that Adobe AIR officially supports iOS 8 and higher (at the time of this posting). It may continue to work with iOS 6 for a while, but don't be surprised if it eventually stops working because that version hasn't been supported in some time.