The filmmakers cannot use the usual disclaimer:
"The story, all names, characters, and incidents portrayed in this production are fictitious. No identification with actual persons (living or deceased), places, buildings, and products is intended or should be inferred."
From what I have seen of the preview, the film "quotes" Joe and Hunter using screenwriter-produced dialog. Those are not quotes.
So how do the filmmakers avoid being sued?
First of all, if what they depict is accurate, there is no basis for a defamation suit. Second, as "public figures" the Bidens would have a steep road to climb to establish invasion of privacy or similar claims, or even defamation claims. Not to mention that there appears to be quite a trove of publicly available court documents relating to Hunter's various depravities.
Although it is also true that anyone can sue, even without a meritorious claim. With the corrupt judiciary that we have, the Bidens might sue. But I think it would be a mistake. The suit would give more publicity to the film and the information that would come out in the discovery process would be stunning.