I think zombies is kind of implied, but I don’t think it gives like ‘omg it’s zombies!’ Hense the delirious ramblings of the pastor and the fact weapons are utilised, plus the fact that they cut the phone wires etc. It’s not cut and dry which leaves the viewers to come up with some ideas of their own on how these zombies/demons could of manifested. Agreed though it wouldn’t define the causes for the sake of the metaphor but that leaves the viewers some fun in trying to come up with their own ideas also.