I really do not know what to really believe about this story. I have seen it before, but logically it would seem clinical signs would appear before the person died, and medical personnel would detect symptoms before the embalmer. But, medically, what do I know? Just a layman.
You haven't been seeing all the recent stories about "sudden death syndrome"?
Something suddenly forms a clot in an important artery, and you will "suddenly" die.