Just because they say "the idea of a talking snake is ridiculous!" does not mean that they rely solely on the absurdity heuristic. After all, they got the correct answer.

"Do not criticize people when they turn out to be right! Wait for an occasion where they are wrong! Otherwise you are missing the chance to see when someone is thinking smarter than you"

If you suspect someone is relying too heavily on the absurdity heuristic, there are absurd things that are true that you can test them on. If you're talking about "the average atheist" and don't have time to test a representitive sample, I would not assume that the absurdity heuristic is all they got going for them.

I'd think the majority, if not nearly all atheists see (some of) the biases that lead to religious beliefs. "All your stated reasons for belief are worthless and shared with the majority, and your claim is absurd" is probably enough to discount the majority.