Defamation requires 4 things:

1. An objectively false statement, portrayed as fact, not opinion.
2. Publication to a third party.
3. Negligence on the publisher, i.e. failure to attempt to confirm the truth of the matter.
4. Actual harm to the victim's reputation and business.

So number 2 is clearly covered as they produced and released a film about it. Number 4 would arguably be covered by the defamation per se doctrine which says that accusation of a crime is de fact harmful to one's reputation.

The problem with the suit is that the film makes so claims of fact. It is disclaimed as a dramatization. A fictional story only based on actual events. Essentially it's historical fiction but with contemporary events as the basis of the history. The characters portrayed are neither named as our based on real cops.

Those things don't necessarily protect them in and of itself, though. It really comes down to whether a reasonable person watching the film would come away thinking the events were fictional or a claim of facts. But I think the general audience is aware how accurate "based on a true story" films tend to be. No one reasonable is expecting the film to be a documentary.