I remember it taking me the longest time to remember the difference between nonfiction and fiction, and between autobiography and biography, when I was in elementary school. It took much repetition of needing to figure out which was which (occasionally I'd look at the shelves of biographies and autobiographies to figure out which shelf was obviously written by the person and which was obviously written by other people) before I really had them straight. I ended up becoming a librarian. That said, that was elementary school, when kids are still solidifying their knowledge of left and right. It wasn't high school, and it certainly doesn't absolve parents (who presumably either graduated high school or at least got GEDs) from having picked it up at some point. I think it's indicative of a non-reader. It isn't important if you don't read.
But, they're good old regular words!
Don't tell me the entire english speaking world doesn't use the words fiction/fictional beyond the fiction shelf of the library?
I know that kids and adults sometime have trouble knowing what is fiction and what is not ("so, did unicorns ever exist? what about Narwhals?", "we sent a robot on Mars, did we sent people too like on the moon?") - but they should still know what fiction means
Just to add, we were visiting friends of the family who are hosting other friends, amongst whom is a 6 y/old boy. We were watching the BBC Who's the new Doctor show extravaganza, and the boy kept asking if it was real or not. I was thinking "if only ><"