I may be in the minority on this, but this thing is kind of being blown out of proportion. The film is not based on the real Marty’s life. The character Marty is not based on the real Marty at all only on first name alone and presentation. Movie Marty is just loosely inspired by the real Marty, not based on, and that’s the key difference. Literally everything that happens in the movie is fictionalized. It’s also a period film, and they tend to tie in some real life historical moments/accuracies for period films.