Copyright of movies covers the video frames but it does not cover the concept of the story. The only part that could be protected are maybe fictional names of characters, cities and maybe special breeds. But the movie does not even mention the word zombie. So anyone would have been able to use the word zombie freely. The general concept of someone dead coming back alive is not protected by copyright simply because general concepts are not protected.
Even if somehow the movie was called ZOMBIE and they copyrighted and patent it, others would have been able to do movies/comics/books WITHOUT using the word zombie.
In the Walking Dead, zombies are called walkers so they would have got around the zombie patent by calling zombies something else.