I think he's saying they aren't Christians because they distort the word of Christ so thoroughly that they don't deserve the title. Anyone can call himself a Christian, but the label means nothing unless the person's beliefs meet certain criteria.
The disagreement is in the criteria.
Bingo! Just because they happen to use the Bible does not make them Christian because they distort Jesus Christ and other scriptures in the Bible. If I stand here and claim something of Jesus Christ and you go look in the Bible and it says something clearly different then there is a problem. Of course they can claim to call themselves Christians, but those who know the truth and read the Bible know they aren't Christians. They are a cult because they distort the word of God and go against the teachings of Christ.
I'm not trying to single out JW's or Mormons, I don't hate them or want to bash them. I'm clearly defining what they are and they are not followers of Christ.