Right.
Death Metal itself is not so much about the vocals but more about the lyrics, which focus mainly on death as a metaphor of good life. Also bands such as Cannibal Corpse's lyrics focus mostly on necrophilia and murder. The band Death went a bit further and incorporated lyrics about philosophy and nihilism. The abrupt tempo and rhythm changes are also a key feature of Death metal, with extremely compley guitar and drum work.
As to it beng instrumental, I'm still to hear a band which is completely instrumental and completely death metal at the same time. Fear Factory is more technical metal, and they do have grunts... or at least the last time I heard them.
Granted, the vocals aren't particularly easy to understand, but you can see their artwork and maybe even read the lyrics in the booklets. The song names also are a giveaway. Like:
- Death (the band): Nothing is Everything, The Philosopher, Lack of Comprehension, Overactive Imagination.
- Cannibal Corpse: ****ed with a Knife, Meat Hook Sodomy, She Was Asking For It, Vomit the Soul.
- Dying Fetus: Born in Sodom, Raped on the Altar, Vomiting the Fetal Embryo.
You get the idea.