I could easily pick this article apart, but I don't see the point. I think it is mostly hogwash. I don't believe darkness can exist in the light, therefore a true Holy Spirit-filled Christian cannot be possessed under normal circumstances. If Jesus taught "Christian demon possession" I am completely unaware of it. I've never seen it in any version of the Bible.
My old church, which I described in earlier posts as charismatic-leaning Pentecostal, taught no such thing that I know of. They taught other highly questionable things that required a certain amount of twisting of the Word to convince folks of its legitimacy, some things I would dare say borders on witchcraft. They held private gatherings that were not open to the congregation in general, which were not administrative in nature, so I can only speculate from that and rumors and other things I saw (and I hate rumors with a passion).
They had a manual entitled "Advanced Spiritual Gifts", which was in 2 parts, of which I saw part 1 only. It included lessons on speaking in tongues, prophesying, praying in the Spirit, faith healing, street preaching (how to put on "The Show"). I don't recall if ever they were challenged scripturally, but they seemed to be able to back up what they did with scripture; it just seemed all too much questionable to me, so I left after 2 years. But again, this was not a totally all-the-way-out-there charismatic church. I play in a touring Christian band and we won't play at one.