Mythology is from the Greek mythos for story-of-the-people, and logos for word or speech, so the spoken story of a people. The idea regarding Genesis is that it's the word of God, not a story created by people.
It's the word of God because it's inspired by God.
But it's not a detailed account of how God created the world, written in the first person.
Somebody else, possibly Moses, wrote about creation. And they wrote in a way that people of the time would understand.
Neither he, nor God, wrote a science textbook or blow by blow account of how stars, the sun, animals, trees, fish, digestive systems etc etc were made.
Gen 1 almost reads like a poem, with it's repetitive verses "then God spoke", "and it was so," "there was evening and there was morning" and so on.
Most books of the Bible are stories. The books of Esther, Ruth and Jonah are the stories of Esther, Ruth and Jonah. Psalms are poetry and yet they tell the stories of factual events. Parables are stories with teach a truth.
All these are still the word of God.
A story is a tale ie fiction.
Not at all.
There are many stories from the war, about monarchs and accounts of history. They are all fact.
I can tell many stories from my life.
The story of deciding to go to college, what happened when I got there and why I left again.
The story of what happened when I got M.E, how people reacted to it and treated me and what I learnt during those 18 years.
I used to tell my nieces and nephew stories about their fathers, and my dad told me stories of his childhood.
All were/are 100% fact.
Genesis is an accounting of what actually happened.
No, it isn't.
It doesn't tell us HOW stars, fish, people etc were made; only that they were.
We can't read Genesis and find God's instructions for creating a plant from nothing. There are no details about how he created animals, trees or people, how he chose colours for the flowers, how he even created colours.
God created the universe; that's all we need to know.
It came into being because God said so - which implies that God wanted it.
It is sustained, and held together, by God's word - see also Colossians 1:17.
There was an order and method to creation - God didn't throw lots of atoms in the air and improvised with what came out.
Of course Genesis 1 is God's word - he spoke, and we had life.
The definition of myth is not 'a description or accounting of what actually happened'.
If Genesis was a factual, scientific account of what happened and how, it would be a science textbook.
It is not.