While the statistic on atheist or agnostic self identification is fairly accurate, I think it is misleading. There is a lot of stigma against these terms in the U.S., and many people who hold beliefs inline with what would generally be considered atheist or agnostic do not self identify as such.
A 2017 Pew Research survey found that 18% of Americans consider themselves "neither religious nor spiritual" and that a further 27% consider themselves "spiritual but not religious" [1]. In total, roughly 45% of Americans consider themselves "not religious". Given the historical trends in the U.S., I suspect these numbers are even higher now.
https://www.pewresearch.org/fact-tank/2017/09/06/more-americ...