Probably not. Chocolate would cause seizures and cardiac symptoms, vomit and tremors, but not massive gut tissue destruction. And I don't remember any parasite doing this. Clostridum will cause diarrhea, not bleeding.

Is probably a poison, but not chocolate (And I'm including radiation under the "poison label" also. There aren't many things that can wipe the intestinal flora from an animal and replacing it by species that create ultra-resistent spores).

Other option would be food contaminated by spores of Clostridum, pesticides or whatever, but I guess that would be easy to spot the coincidence of a particular brand and batch of dog food in all affected animals.

I thought chocolate was toxic to all dogs.

It is toxic to all dogs.

To be fair, chocolate is not what it was. Many cheap substitutes on the market

Still toxic.

Never give chocolate to dogs! That's poisonous. Or to rabbits. Don't let them eat cake either.

Don't be murderously disservant. It's torture for animals to be served with things that fastly kill them. All of those things do.