I doubt that it was IBD.
Snakes often seem to flop about at death. They are usually found twisted up in odd poses.
The mouth rot may have been a symptom of a more widespread infection instead of being the only thing wrong. Unfortunately reptiles hide illness so well that we often do not see anything wrong before things are severe.
Disinfect everything. I know that an exotics vet is extremely pricy, but it can also be a needed expense, the same as a standard vet if your dog got hit by a car accidentally. You may want to think about saving up an emergency fund before you purchase a new animal, to give yourself peace of mind. That way if something did happen, you would be prepared instead of having to second guess about things.
Without a full necropsy, there's no way to tell what actually killed the snake though. And unless the body was refrigerated and sent off in a timely manner, you can't get a necropsy after the fact very easily.