There's a difference between explicitly closing off any potential for liability and what's being described. I could, in fact, write a story that I describe or s

There's a difference between explicitly closing off any potential for liability and what's being described. I could, in fact, write a story that I describe or strongly imply as fictional (this is the essential part) about anyone wherein I describe them as doing heinous or illegal acts. This is why satire is legal, this is why e.g. John Oliver can describe clearly untrue things about Trump or whomever for the sake of a joke, why comedy podcasts can make jokes about individuals that no reasonable person would presume to be true. The reason that movies have that disclaimer is: so they can avoid even the chance of people saying the movie's describing their real (non-fictional) life (which might require payment on the part of creators of the movie) or (by virtue of describing their real life) open the movie up to libel claims if a reasonable person may presume the work is accurately describing that person. They're crossing their T's because lawyers are expensive whereas comedians to some extent thrive on drama like that (because they almost certainly won't be held to libelous).