I'm familiar with both quotes, but I think we're barking up the wrong tree by trying to intepret them literally.
"Based on a true story" is not what makes a film plausible to us... We instinctively engage in a willing suspension of disbelief when we're prompted by literature, even when we directly know that the story is false.
Fargo includes that text because the film is an homage to the hardboiled crime fiction genre, which frequently featured that style of blurb on book covers as a marketing tool. But people consume books & films, near universally where we have the means to do so, with or without claims that the stories within are factual.
You've misunderstood what Nicola Morgan is talking about, in that quote. In context, "believable" means that the author's job is to reduce obstacles that get in the way of our willing suspension of disbelief... Empathy, context, proportionality, etc.
See my other comments, re: the David Lynch quote. His meaning needs to be taken in context of some film theory.