>this polytheistic culture has a creation myth that is similar to the book of Genesis, therefore christianity is polytheistic
Kek.
All civilizations live on planet Earth, and there are some features that are common to most or all areas of the planet. It shouldn't be a surprise that two different cultures had similar myths or religious concepts.
For example, snakes are recognized as enemies in basically every religion in the world, not because those religions are copies of each other, but because snakes are very widespread throught the planet and are very deadly.