From my brief reading it sounds like the person writing the memo may not be able to distinguish between surveillance of the carriers and putting backdoors into the devices themselves. I can believe that the carriers would be compromised, sooner than I'll believe that the operating systems are.
Specifically, one of the things Apple did with iOS 5 for iCloud was put in end-to-end encryption into the system. iOS has for an even longer period contained protections to make it less easy for an app to read data belonging to other apps, and the storage on Flash has been encrypted since at least iOS 4 (I believe.)
Yet the iOS 5 binaries are subject to the scrutiny of the jail breaking community, and if there is a backdoor here, I'd think that it might have been found. (Or will be soon now that its existence has apparently been leaked.)
Further, how could this actually work? (the memo is so full of jargon and nonsensical to me that I stopped trying to interpret it pretty quickly.)
Its tricky to get intentional communication- FaceTime, iMessage, etc, working on devices, let alone back doors for a specific government. I don't think the indian government could successfully delver to Apple a spec for them to implement that wouldn't intrinsically cause noticeable problems and thus reveal its existence regularly.
In short, I suspect that a backdoor is kinda infeasible from a software reliability viewpoint.
Finally, its also quite possible that this is a psyops campaign. By announcing that they have a back door (thur a leaked memo) the indian government could be attempting to bring pressure to bear from other governments to get their own backdoors, and thus, enable india to ultimately actually get a backdoor. I remember months ago reading that the indian government wanted more access to the mobile devices but were being stymied by manufacturers.
I've been observing Apple fairly closely for over 30 years and working with their operating systems, often at a fairly low level for almost 20 years. I've never seen Apple do anything that would betray their customers trust. People may object to the decisions Apple makes, but I have never seen a decision that, at the end of the day, didn't have at least the intent to do right by the customer (even when they failed to achieve that intent.)
Thus I'm highly suspicious at any claim that Apple has put in a backdoor. I think this kind of claim is extraordinary and out or character, and Apple has far more integrity in my eyes than these documents.
So, I'd like to see some evidence before we accept that this is fact.