... But short of a mental issue, how does one 'believe' a story that they know is not true?
Okay, Dude Man, I was having fun with ya, but now I'll give ya a more serious answer.
Yes, how does one believe a story that they know is not true. Mental illness probably plays a role, but how - eh?. I've got a few answers that might shed some light on that dynamic. One is Disassociative Identity Disorder, and that is a mental illness. However, in DID one personality believes the story and the other personalities do not, because it is not their reality. D.I.D. is real and it's real trippy to witness. None of the personalities look or act "crazy," necessarily, but they might. Perhaps Rodney Bonnifield has DID and tells his DB Cooper story in one of his personas.
#2. Rodney was always pictured in the film drinking a beer. I think it's fair to say he is an alcoholic. Maybe the alcohol has altered his neurological structures sufficiently to give him a kind of self-created psychosis, in which he sees himself being DB Cooper. In effect, he has imagined his life as DBC into being. He lives in a delusion; quite comfortably, too - probably staying drunk 24 hours a day to maintain the delusion.
#3. Another possibility is a quantum shift. Rodney wants to be DB Cooper so bad he has become DB Cooper. Most quantum physicists that I know would poo-poo this notion, but there is nothing in quantum theory that I know of that prevents such a manifestation. In fact, Rodney's reality shift into Cooper-ness may be produced as a very real functional aspect of the Many World Theory of Reality, which is espoused by most physicists. Ironically, the physicists believing the Many Worlds theory may not believe in some of the potential consequences of that theory - if it's real.
#4. A related dynamic may be sociopathy. Rodney may want to be famous so bad that he acts out the DB Cooper story to fulfill his inner desires. His compulsions may be so strong that the truth of who he is fades into meaningless-ness and he is able to shift into Cooper-ness and "act" as DB Cooper convincingly, just as sociopaths bamboozle their way through life. Similarly, maybe Rodney likes being a super-duper con man. Maybe he slides into Cooper-ness just to goof on people. Clearly the filmmakers are quite gullible, and maybe Rodney was having his kind of perverted fun.
#5. PTSD. Rodney's life may be filled with so much trauma that his mind retreats to the safety of being DB Cooper. It's a kind of psychosis, but real, certainly in terms of believing it.
Lastly, besides DB Cooper, everything else in Rodney's life - the thuggery, the women, the drugs, the violence - may all be untrue. Further, it could all be sourced to some of the above.
Lastly, lastly - looking at the majority of Russians these days, they believe Putin and believe all the destruction in Ukraine is fake, even when their family members call them from Ukraine and tell them the "reality" of what is going on. Such is the power of social conditioning. Hence, I think is is fair to say that propaganda works and that state-sponsored brain-washing is real. So is MKULTRA.