It seems like you are confusing “metaphor” with “fiction”.
The question is whether or not the Flood narrative should be taken as a factual historical account, or as a story that is mostly intended to teach a theological truth.
It is also not true that narratives always fit into nice fiction/non-fiction binaries, where in the former category everything is objective fact and in the latter category everything is pure imagination. Authors embellish narratives and re-frame actual people and events to fit their story-telling purposes all the time. It’s like asking if the movie Apollo 13 is true or false, fiction or non-fiction.
We need to think about what the story intended to communicate. When Jesus told the story of the Good Samaritan, no one was confused and thought he was teaching them “literal history,” even though there are plenty of details. Most Christians reading the account of the anointing of David by Samuel on the other hand, assume it is an account of what really happened to a historical figure. So we need to figure out whether the point of the account of Noah was to record facts about history, or something else.
The fact that very parallel accounts of a great Flood exist in the ancient near East cultural milieu, the fact that science clearly indicates we did not all descend from 8 people a few thousand years ago and there is no geological evidence of a worldwide flood, and the fact that the themes of the story (impending judgment, people who won’t repent, God providing a means of salvation, God (re-)creating a world out of watery chaos) are important theological themes might be clues that the main point was to teach theological lessons, even if some of the people really lived and even if there was a real flood that inspired the story.