Unless Chelsea Manning is trying to convince us to trust her with our private data, I don't see where the irony is.
It's similar to a reformed/retired safe cracker is telling us which safes are hard to crack. The safe cracker would be in a position to know about how difficult it would be to crack safes.
The major difference being that the safe cracker possibly stands to benefit from lying to us in an effort to make their job easier (convince us to get safes that are in reality easy to crack). In the situation with Chelsea Manning, I'm not sure where you think that she stands to benefit from us following any advice that she's giving.