I also want to have true beliefs about Lewis and about Lewis's writings.

Judging authors by a single quote without knowing the context is a bad idea. Rousseau begins one of his works by saying ""Man is born free, and everywhere he is in chains."

Taken on it's own you might think that Rousseau is somehow criticizing that man is in chains. He isn't. He advocates that man is chained by the social contract. I really had a hard time with ideas like this because I had read the book and my history teacher hadn't, so discussing Rousseau was really hard.

Why do you care about Lewis?

A proper rationalist quote should only be about its apparent subject and should not try to sneak in such an attack under the radar.

We are a group of smart people. There nothing wrong with a quote having multiple layers of meaning and saying something in addition to it's apparent subject.

Nobody has the time to evaluate all of them; we must pick and choose between them.

On LW we care about rationality. Having accurate beliefs about what makes people become atheists is useful for that purpose.

On the other hand having accurate beliefs about CS Lewis is less important.

The implicit attack "all materialists are materialists because they are flawed humans who make mistakes" also has a truth value, and this truth value is something that I can care about just as much as the truth value of the statement's literal words.

The truth value of that statement matters a great deal but that in no way implies that we shouldn't take about that statement and it has no place on LW.

You didn't attack it on grounds that it's wrong and that there evidence that it's wrong but on the grounds that it's an unfair attack.