Yes, religious Americans - many of them - are that stupid. They hardly think about their beliefs at all, and when they do, they turn a blind eye to legitimate criticisms of their beliefs.
They believe that their eternal salvation rests on affirmation of certain "right beliefs" and in a "right relationship" with God. Anything that challenges their spiritual status quo is really Satan in disguise, because to question the truth of their religion is to question the religion; is to question God's having "ordained" or "instituted" that religion; is to question God; is to commit the unforgiveable sin; is to lose God and salvation and the hope of Heaven.
For these kinds of believers, therefore, there is a built-in defense mechanism for avoiding truths, attitudes, and ways of thinking that would in any way question the core beliefs.
There is a built-in mechanism that supports such mindless credos as "God said it; I believe it; that settles it".
Moreover, the religious dogmas as usually presented, are not designed to withstand much critical evaluation. It is so much easier just to "take it all on faith".
Which, to my mind, is a primary reason why so many religious Americans flunked the religious literacy test. They are scared and intellectually lazy.