If they can find a flaw in AES 256, others can too.

My theory is they've got a backdoor in the iPhone hardware random number generator. It's the most obvious high-value target, it's inherently almost undetectable (the output of a CSPRNG is indistinguishable from real random numbers), and you can keep crypto researchers busy with fights about whose cryptosystem is best, safe in the knowledge that it doesn't matter what they come up with, they were owned from the start.