It's only a no true Scotsman fallacy if a Scotsman is not well defined. In their minds, what is and isn't a christian is clearly defined and they do not fit the criteria. I would say that a better example of the fallacy would when Christians say that people that were once members of their church and people that the once accepted as Christian, suddenly were never a Christian because they lost their faith.
Although, even then, they could be defining a "true Christian" as someone who is faithful for life, and therefore no one who lost their faith would ever be a true Christian. That also though, would have the side effect that only those who died professing Christianity are true Christians, so no Christian alive is a true Christian either.