True, and an important distinction. Christians can still be demonized (simply having the Holy Spirit indwelling the temple of your body no more prevents demons from entering than having the glory of God in Solomon's Temple kept the Jews from populating it with idols. However, we cannot be totally possessed by the Enemy, being already the possession of Christ.
But that doesn't mean that demons cannot cause severe problems or that Christians should be under the belief that they cannot be attacked or even indwelt by the demonic if they do not take the authority Christ gave them to kick the demons out.
Hence the difference between possessed and oppressed. I cannot as a Christian be possessed. Jesus already owns me and He's not giving me up. I can however, if I get real stupid, invite things in that I really don't want in. Thus ending up oppressed by those things. Again subtle but important difference. I've had it explained to me that a possesed person has the demons in them controlling them while an oppressed person has the demons outside of them strongly influencing them (the old devil on the shoulder thing)