First, define a Christian ;p It actually isn't as simple as you'd think - for example, consider the definition this site uses:
As far as the scope of this site is concerned, any group that identifies themselves as Christian are to be considered on-topic and allowed to label themselves Christian.
You would perhaps need to ask the author, but "cultural Christians" may be one answer; for example, the recent results of a UK MORI (i.e. well-respected/independent survey) of only people who self-identified as Christian showed that 65% are actually non-religious cultural Christians, i.e. identify as Christian for reasons such as "was Christened as a child", "parents are Christian", etc. Likewise, only 10% used religion as a primary tool in morality issues. Summary document - but emphasis: while the survey was commissioned by the Dawkins Foundation, the survery was conducted independently paying scrupulous attention to the actual questions to ensure the questions weren't leading, etc.
I can't know whether this relates to the thoughts of the answer you are thinking of, but that number is pretty telling, particularly when interpreting geographic religion statistics - i.e. how people identify might not actually relate much to their religious beliefs, but more to their historic/cultural identity.