I don't think anyone has said that we are able to say "that person isn't saved", I think we've all been clear that only God can make that judgment. What was said was that doubts can be put into our heads about certain people. I would never say to someone "You are not a true Christian"- I would never want to make that assumption. That is condemnation. If we condemn, then we are condemned.
About judging- although Jesus didn't actually tell us we can't judge at all, He did say that we have to careful about how we judge- which, to me, is the meaning of what He said. Since none of us is perfect, then we do need to be careful about how we judge others. We all judge- we judge whether or not we want to be a close friend to someone, whether we can trust someone with a secret, and minor things of that nature. If someone judges another person for being a drug addict and the person is an alcoholic, then that would be an example of how we are not supposed to judge.