Blatant ad hominem?

Papalinton seems to be arguing as follows:

1. C. S. Lewis defended the view that there is a God, and that Christianity is true.
2. But C. S. Lewis was a fantasy fiction writer.
Therefore, his arguments in defense of belief in God and Christianity cannot be taken seriously.

Now, if this isn't what he's arguing, I've seen plenty of people say the same thing.

Can anyone explain to me why this is not a blatant example of the ad hominem fallacy? Why would a fantasy fiction writer be less likely to defend true propositions in the area of religion, as opposed to a writer of detective fiction, or a or realistic novels, or science fiction, or what have you, or even than people who don't write fiction?

If this were just Papalinton, I probably wouldn't devote a post to it. But I've seen it from a quite a few people.