I'm not really sure why you think this needs substantiation. The logic goes like this: It would be utterly stupid for terrorist groups to rely on digital comm

I'm not really sure why you think this needs substantiation. The logic goes like this: It would be utterly stupid for terrorist groups to rely on digital communications simply because of how well locked down the various government agencies have made it. We assume that they are not utterly stupid. I don't think it's a crazy statement or needing further justification. I doubt terrorist groups are that sophisticated but I would assume they have plugged the simplest holes.