Genesis is a handwritten historical account of the creation (God being the eyewitness till day 6), the flood (Noah, Shem, Ham, Japheth & their wives were the eyewitnesses) and the geneology's handwritten accounts (so & so begat so & so) of at least 9 different authors for the first few chapters of Genesis. Moses, being EXTREMELY well educated & having a plethora of "ancient original texts" from the Egyptian libraries compiled the eyewitness accounts & "wrote" the entirety of Genesis as one complete book. Most scholars have no issue questioning whether Moses wrote the other 4 (Exodus, Leviticus, Numbers, Deuteronomy).

Anyway, long story short, OP is correct. There was no religion before "the beginning" (obviously because you can't have something before nothing) and there was no other religion (we can even call it monotheism if you prefer) immediately after the flood.