The point is that you don't ignore countless people saying the same thing just because you can think of a reason to dismiss them. Even if you are right and that's all it is, you'll still have sinned for not considering it.

Doesn't the very fact that I have a reason imply that I must have considered it?

And at any rate, how is "They got their ideas about rationality from popular fiction" a failure to consider? Things are not always said by countless people because they have merit. And in this case, there's a very well known, fairly obvious, reason why countless people would say such a thing. You may as well ask why countless people think that crashed cars explode.