I guess I don't find this surprising but it is somewhat irrational. About 90% of self proclaimed atheists agree with the proposition:
The Bible is a very unreliable record of ancient history.It may be anticipated that atheists do not give the Bible much credibility concerning its theological teaching—else they might be theists. And it probably is not unexpected they disagree with the first few chapters of Genesis given that abiogenesis and macroevolution is the major competing worldview, and Darwin is the darling of atheists worldwide. But to insist that Bible history is generally unreliable, let alone very unreliable, reveals an irrational anti-biblical bias.
Other ancient texts with obvious internal errors are often treated as generally reliable. People read Herodotus, Plato, Thucydides, and Manetho; and refer to records of ancient nations; and consider the material reliable, or at least give it the benefit of doubt. These ancients write with their own agendas, with religious claims, and often times refuse to document the failings of favoured men. At minimum it seems reasonable to give the biblical historians a similar standing—especially given they frequently document the failings of their people and leaders!
Further, the archaeological confirmation of several biblical claims should increase the biblical authors' secular credibility.
I think this atheistic response originates not from the realms of reason for which they pride themselves, but is the emotional reaction of their intense antitheism.