That's assuming the NSA didn't hack it. Just because we know about a failed attempt doesn't mean they didn't succeed on a different try.NSA attempted to hack Alexa and it can only be listened into when the user accesses it.
If the NSA can't do it, I think it's hard for anyone to.
That's also assuming that they even need to hack it and that Amazon hasn't given them a back door. It's not like companies haven't done that before and Amazon already played nice with law enforcement when it comes to cameras.