Why do atheists say just because a WRITTEN account is older that instantly means that religion?

The older texts, of themselves, do not make polytheism older. The repeated emergence of monotheism from polytheism is sound evidence. Even looking at the older Hebrew of the Bible show that there was a polytheistic era. The use of plural forms for the name of God is evidence that Judaism arose from polytheism. There is overwhelming evidence for polytheism to monotheism, and none for the reverse.